In 1997, rock group Sharam Q, fronted by Tsunku, began auditions for a female vocalist. These auditions aired via the reality show ASAYAN, and resulted in Heike Michiyo being crowned the winner. Tsunku, however, decided to give five of the runner-ups—Nakazawa Yuko, Abe Natsumi, Iida Kaori, Fukuda Asuka, and Ishiguro Aya—a chance to become a group in their own right, and named them Morning Musume. The girls were set the task of selling 50,000 copies of "Ai no Tane", their demo single, in just five days. They sold the required number of copies in four days, and subsequently became an official group.
On January 26, 1998, Heike Michiyo and Morning Musume opened an official fanclub together called Hello!. Just two days later, Morning Musume's debut single, "Morning Coffee", was released under the One Up Music label and charted at #6 on the Oricon weekly chart. In August 1998, Morning Musume and Heike Michiyo held their first official joint live under the title Hello! First Live at Shibuya Kohkaido. Morning Musume's first official unit Tanpopo was formed in late 1998, and their second official unit Petitmoni was formed in late 1999.
In 1998, the Morning Musume Tsuika Audition took place, resulting in the addition of "second generation" Morning Musume members Ichii Sayaka, Yaguchi Mari, and Yasuda Kei. In November that same year, the Geinoujin Shin Unit Audition began on ASAYAN and the Pacific Dream Pop Singer Contest began in Hawaii. Taiyou to Ciscomoon and Coconuts Musume would be formed the following year in 1999 from the winners of these auditions.
In March 1999, auditions for Country Musume were held on the TV Tokyo show Idol wo Sagase!. At the same time, the 1st Morning Musume & Heike Michiyo Protegee Audition was held, resulting in the addition of Miyoshi Chinatsu as a soloist.
In April 1999, the official fanclub was renamed Hello! Project. A month later in May, the 2nd Morning Musume & Heike Michiyo Protegee Audition took place and resulted in the four-member group Melon Kinenbi.
In July 1999, Hello! Project's first live concert Hello! Project '99 was held at Yokohama Arena.
In March 2000, the first Hello! Project shuffle units were formed. Shuffle units would be formed every summer up to 2005 (with the exception of 2004), after which the concept was abandoned in 2006.
In May 2000, Matsuura Aya became a soloist after participating in the 4th Morning Musume & Heike Michiyo Protegee Audition. Kitagami Ami and Oki Ibuki participated in the same audition, and went on to form Sheki-Dol the same year.
In March 2001, Morning Musume co-founder and leader Nakazawa Yuko announced her graduation from the group. The actual graduation took place on April 15, and Nakazawa was subsequently named as leader of the entirety of Hello! Project. Nakazawa's graduation garnered considerable media attention, the first time the group had truly caught the attention of the mainstream press.
Sheki-Dol announced they were going to disband in early 2002 due to a member needing medical care.
Between April and June 2002, the first and only Hello! Project Kids auditions were held. These auditions resulted in acquiring the future members of Berryz Koubou and ℃-ute. It also spawned off-shoot auditions such as auditions for Hello Pro Kenshuusei and Hello Pro Kansai.
On July 31, 2002, a major reformation of all of the groups within Hello! Project began, which was referred to as "Haromageddon" (ハロマゲドン) (a pun on "Hello" (ハロ) and "Armageddon" (ハルマゲドン)) and "the 7/31 incident" by fans.
In February 2006, former Morning Musume member and sub-leader of duo W, Kago Ai, was caught smoking in public while underage and dating a much older man. Kago was suspended from her activities and eventually began an attempt to return. On March 26, 2007, Kago was again caught smoking and her contract was instantly terminated.
In June 2007, it was announced that Konno Asami, who had graduated from Hello! Project a year prior to study at university, would be returning. Konno became the first person to have graduated from Hello! Project to return.
On October 28, 2007, former Morning Musume member Goto Maki, who had been inactive for some time, graduated from Hello! Project as a soloist. She later transferred to Avex Entertainment in June 2008.
In April 2008, the debut of Hello Pro Kansai group SI☆NA was announced. The group never officially debuted and never recorded a song, though they occasionally appeared on television shows and radio.
In September 2008, it was announced that, as a result of the Taiwanese auditions, the six-member group Ice Creamusume and the duo Frances & Aiko, both still only children, were formed under Hello Pro Taiwan.
2008 also marked the first time in a decade that Morning Musume had not performed at the annual NHK Kouhaku Uta Gassen event, after ten consecutive appearances. The only Hello! Project member to appear on that year's show was Satoda Mai, marking the first appearance of a Country Musume member.
In February 2009, it was announced that Hello! Project would begin a series of auditions in South Korea. The sole winner was Jang Da Yeon, who only participated in Hello Pro Egg activities before withdrawing sometime after 2011.
On February 1, 2009, at Yokohama Arena, Hello! Project held its largest concert ever, the Hello! Project 2009 Winter Kettei! Hello☆Pro Award '09 ~Elder Club Sotsugyou Kinen Special~, featuring 21 groups and 72 members. During the concert, then-Hello! Project leader Nakazawa Yuko passed her leadership position to Morning Musume leader Takahashi Ai.
Later in 2009, several of the old, dormant units were revived. Tanpopo, Minimoni, Petitmoni, ZYX, Aa! and v-u-den all returned with new line-ups, and High-King returned from hiatus with its original members. These groups became known as "Chanpuru" units.
After Niigaki Risa's graduation from Morning Musume and Hello! Project in May 2012, Michishige Sayumi took over as leader of Morning Musume and Hello! Project. Mitsui Aika also graduated from Morning Musume and stayed in Hello! Project as a solo act. That same month, Hello Pro Egg also changed its name to Hello Pro Kenshuusei.
In 2012, UP-FRONT AGENCY created the SATOYAMA movement to promote environmentalism and preserve culture in rural areas. It spawned several units consisting of Hello! Project members and other talent under UP-FRONT GROUP such as Peaberry, DIY♡, Harvest, and GREEN FIELDS, as well the creation of the TV show Hello! SATOYAMA Life.
On January 30, 2013, the YouTube channel Hello! Project Station opened and would deliver news, music video previews, concert performances, behind the scenes, and other projects featuring Hello! Project members weekly.
On February 3, during the Hello! Project Tanjou 15 Shuunen Kinen Live 2013 Fuyu ~Bravo!~ concert in Fukuoka, a new unit was announced. Its founding members were Miyazaki Yuka, and Hello Pro Kenshuusei members Kanazawa Tomoko, Takagi Sayuki, Otsuka Aina, Miyamoto Karin and Uemura Akari, with each member representing a fruit. On February 25, their member colors and the unit's name Juice=Juice were revealed.
In March 2013, UP-FRONT AGENCY introduced the SATOUMI movement to co-exist with the SATOYAMA movement, but instead focus on sea life and coastal communities. The units Dia Lady, Plumeria (later renamed HI-FIN), and Mellowquad were announced as part of the new movement. A new SATOYAMA movement unit named Jurin was also announced in July 2013.
On July 26, 2013, Hello! Project changed its logo. The new logo had a navy blue background, which would later change to a brighter blue color, with the white text "Hello! Project" in middle and "since 1998" underneath it.
On August 2, 2014, during the Hello! Project 2014 SUMMER ~KOREZO!~ concert, Shimizu Saki announced that Berryz Koubou would go on indefinite hiatus after the conclusion of their spring 2015 tour.
In October 2014, after the Morning Musume '14 Live Concert in New York, Tsunku resigned as Hello! Project's general producer in order to rest and spend more time with his family after his vocal cords were removed due to laryngeal cancer. He would continue as one of UP-FRONT WORKS' songwriters and remain as Morning Musume's sound producer, but would decrease his amount of work. Tsunku's resignation was unknown to public until September 2015, when he confessed to it in the memoir Dakara, Ikiru. he had released.
On November 5, 2014, it was revealed that Country Musume would be revived as Country Girls, with a new line-up of two Hello Pro Kenshuusei members, three Morning Musume '14 audition participants, and Tsugunaga Momoko as the playing manager.
On November 26, 2014, Michishige Sayumi graduated from Morning Musume '14 and Hello! Project at Yokohama Arena. At the time of her graduation, she was the longest serving member of Morning Musume. ℃-ute leader Yajima Maimi became the new leader of Hello! Project.
On December 17, 2014, following the announcement made in September that they would add third generation members and also be changing the group name, S/mileage's new name was revealed to be ANGERME.
On January 2, 2015, a new unit made up of eight Hello Pro Kenshuusei members was announced during the Hello! Project 2015 WINTER ~DANCE MODE!~ concert. On February 25, the group's name was revealed to be Kobushi Factory. The name was chosen with the help of Shimizu Saki, who wanted them to carry on the spirit of Berryz Koubou.
On March 3, 2015, Berryz Koubou performed their last concert before their indefinite hiatus, Berryz Koubou Last Concert 2015 Berryz Koubou Ikube!, at Nippon Budokan. They had been a group for 11 years, 1 month, and 17 days and graduated on the 11th anniversary of their debut.
On April 29, 2015, another new unit composed of six Hello Pro Kenshuusei members was announced to Japanese media and on Hello! Project Station. The group name Tsubaki Factory was once again chosen with Shimizu Saki's help, who wanted them to also pass on the spirit of Berryz Koubou as a sister and rival group to Kobushi Factory.
On July 21, 2015, Hello! Project officially opened their first official smartphone app, Hello! Project Mobile or "Hello! Moba" for short. The 400 yen monthly subscription plan allowed users of the app to gain access to exclusive content such as "Hello! Gacha" or the popular Pocket Morning questionnaires. Users with the 700 yen monthly subscription plan could also gain access to "Hello! Radio".
On October 29, 2015, Sayashi Riho announced her graduation from Morning Musume '15 in order to study English and dance abroad, but would stay as a member of Hello! Project. Following her graduation on December 31, she was listed as a Hello! Project solo act.
On August 20, 2016, ℃-ute announced they would disband in June 2017 after a concert at Saitama Super Arena. On November 5, 2016, fellow former Hello! Project Kids member Tsugunaga Momoko announced that she would also retire from entertainment in spring 2017.
On December 31, 2016, at the Hello! Project COUNTDOWN PARTY 2016 ~GOOD BYE & HELLO!~, Yajima Maimi announced that ANGERME leader Wada Ayaka would become the new leader of Hello! Project at the start of 2017 with Morning Musume '16 leader Fukumura Mizuki as the first ever sub-leader of Hello! Project. Wada is the first former Hello Pro Egg to hold this position.
On June 9, 2017, it was announced that Country Girls would change to a new system after Tsugunaga Momoko's departure. The group would become semi-active with three of the group's members joining other Hello! Project groups as concurrent members for their main activities. An announcement video was later uploaded to the Hello! Project Station YouTube channel, revealing the new placements of the transferring Country Girls members and a few promoted Hello Pro Kenshuusei, including a new group led by Ichioka Reina.
On June 12, 2017, ℃-ute held their last concert, titled ℃-ute Last Concert in Saitama Super Arena ~Thank you team℃-ute~, a day after the 12th anniversary of their formation. A few weeks later, Tsugunuaga Momoko graduated from Country Girls and Hello! Project at the Tsugunaga Momoko Last Live ♥Arigatou Otomomochi♥ on June 30, 2017, the day of Hello! Project Kids' 15th anniversary.
On September 8, 2017, Inaba Manaka announced her return to Hello! Project after a year and half long hiatus due to health issues, which resulted in her graduating from her debut group Country Girls during her absence. She would advise Hello Pro Kenshuusei Hokkaido as their new leader, but also pursue individual activities as a Hello! Project solo act.
On October 23, 2017, a new YouTube channel titled tiny tiny was announced to start on October 26. The hosts are Sharam Q's Makoto, who is also known for being the longtime MC of Hello! Project concerts, and Kato Noriko. Each week, an active Hello! Project member or OG member is brought on as a guest to deliver "small pleasures that would unexpectedly become a great source of happiness and brighten the lives of viewers".
On February 28, 2018, an Instagram campaign, titled "Hello Pro here" (or "H!P here"), was announced in order to grow closer with international fans. Fans have been asked to take photos and videos of famous attractions and famous products where they live, while also showing their love for Hello! Project, and to post it on Instagram with the hashtag "#hp_here". Out of all the posts shared, one person will be randomly selected to appear on Hello! Project Station and speak with a member. The campaign also allows fans to promote their cities for concerts to take place there in the future. A month prior to this announcement, fans outside of Japan have also been encouraged to send in questions for Hello! Project Station's MC talk corner.
On May 1, 2018, staff announced that a new web show based on viewer feedback was being planned for summer 2018. It would include future projects with international fans similar to the "#hp_here" campaign on Hello! Project Station. The show, which was later announced as the OMAKE CHANNEL, began on August 1, 2018.
On May 11, 2018, the Hello! Project "ONLY YOU" Audition began, searching for members of a new group led by Ichioka Reina and a second new group including Takase Kurumi and Kiyono Momohime in its line-up.
On October 19, 2018, the two new groups were named CHICA#TETSU and Ame no Mori Kawa Umi, and it was announced that together they would form the combined group named BEYOOOOONDS alongside the Hello! Project "ONLY YOU" Audition winners.
On December 7, 2018, it was announced that another solo talent, Sayashi Riho, who had been on hiatus since she graduated from Morning Musume at the end of 2015, decided to graduate from Hello! Project at the end of November when her exclusive contract with UP-FRONT PROMOTION had ended.
On December 31, 2018, during Hello! Project 20th Anniversary!! Hello! Project COUNTDOWN PARTY 2018 ~GOOD BYE & HELLO!~ first part, Wada Ayaka announced that Fukumura Mizuki would become the new leader of Hello! Project after her graduation.
On March 30, 2020, Kobushi Factory disbanded.
On May 11, 2020, an official Hello! Project TikTok account opened up, and select songs from the current active groups were made available to use for editing in the app.
On June 15, 2020, after months of concerts and events being cancelled due to the COVID-19 pandemic, UP-FRONT PROMOTION announced their plans for the annual summer concerts. Unlike previous years, the Hello! Project 2020 Summer concerts featured the members performing solo covers of J-Pop ballads by artists outside of Hello! Project. In compliance with government policy, cheering, crowding, and closeness in the venue are prohibited, the venues were filled to 50% capacity, and wearing masks was required to enter. They made the decision to hold the concerts despite the pandemic due to the members' desire to stand on stage as soon as possible.
On February 10, 2021, the special compilation album Hello! Project no Zenkyoku kara Atsumechaimashita! Vol.7 was released as part of a seven part series. Unlike the previous six volumes, which were all released in 2014, this volume commemorated the theatrical release of the 2021 film Ano Koro, a movie based on a true story of the life of a Hello! Project fan and the camaraderie that fans form in their passionate support of Hello! Project idols.  Ano Koro was released in theaters on February 19, 2021.
On March 12, 2021, the official HELLO! PROJECT STREAM ONLINE STORE website opened. Through the site, fans are able to purchase streaming tickets to access digital video recordings of concerts, events, or other shows featuring Hello! Project or M-line club acts. These recordings are not streamed live, and are available for repeated viewing during a set period of time.
On July 5, 2021, it was revealed at Hello Dream. that the Hello! Project Shin Member Audition 2021 (ハロー！プロジェクト 新メンバーオーディション2021) will start on July 20, 2021, in search of new members to join the new Hello! Project group.