In 2013 Pledis Entertainment began the regular live broadcasting of a program called 17 TV on the online streaming platform UStream. During various broadcasts, fans were invited to watch Seventeen as they trained in their practice room. The show had different 'seasons' in which Pledis unveiled their trainees to the public,some ending in their own Like Seventeen Concerts
On April 19, 2015, Pledis Entertainment began releasing teasers for Seventeen's first reality show Seventeen Project: Big Debut Plan on a major broadcasting channel, MBC Entertainment. The first episode premiered on May 2, 2015 and the show ended on May 26 with the live debut showcase.
Seventeen debuted on May 26 with a live showcase: the first male K-pop group to debut with a 1-hour live showcase on a major broadcasting channel. Labelmates Lizzy and Raina from After School served as MCs. Three days later the extended play 17 Carat was digitally released along with the music video for their title track "Adore U" (아낀다). The physical album was later released on June 2. Despite the group originating from a relatively small company, the album ranked ninth on the Billboard World Albums chart a week after its release and peaked at eighth, staying on the chart for a total of eleven weeks. 17 Carat later ranked 9th on Billboard's list of the 10 Best K-pop Albums of 2015, the only rookie album on the list.
After the release of teasers for Seventeen's second mini album, including the track list and two versions of album packaging, their second extended play titled Boys Be was released on September 10 together with the music video for the title track "Mansae" (만세). Pre-order sales for Boys Be surpassed 30,000 album copies, with further sales confirming the group's popularity, as it became the highest selling rookie album of 2015. The album debuted at number one on Billboard's World Albums Chart. The group was listed as the only K-Pop group under Billboard's 21 Under 21 chart for the most promising young artists of the year, and also earned them various awards, such as the Golden Disk Awards and the Seoul Music Awards.
On December 4 a digital single titled "Q&A" was released by Seventeen. The single featured members S.Coups, Woozi and Vernon, who were joined by singer Ailee. The track was revealed to have been created the previous year by Woozi, long before the group's debut.
Seventeen held four concerts entitled 2015 Like Seventeen – Boys Wish from the 24th to the 26th of December as an end-of-year celebration at Yongsan Art Hall, Theatre Mir. All tickets sold out within one minute. After the concerts' success, Seventeen held two more concerts on February 13 and 14 entitled Like Seventeen – Boys Wish Encore Concert which sold out in five minutes. At the second concert the official fanclub name Carat (Hangul: 캐럿) was announced.
Seventeen's first studio album named Love&Letter was released at midnight local time on April 25. The album comes as both a Love and Letter version much the same as the Boys Be album had two separate versions. The album sold over 150,000 copies prior to release, more than quadruple the amount of pre-orders Boys Be had, and ranked in Japanese stores despite there being no Japanese promotions. The music video for the title track, "예쁘다 (Pretty U)", was released on April 24 after the group's showcase.