2015-present: Success with first fully self-composed album and pursuing a new genre

They started the year with their World Tour called 2015 FTISLAND [FTHX] Tour which included concerts in France, Brazil, Chile, Mexico and two dates in the United States, one in New York and one in Los Angeles.

After a year and four months of hiatus in Korea following their mini album, The Mood, FNC Entertainment announced F.T. Island's first fully self-composed album, I Will which was released on March 23. The music video of "To The Light" was pre-released on YouTube on March 18.[105] The album peaked at 7th on the Billboard World Albums Chart in the second week of April.[106] The title song "Pray" was also rated 99.0 out of a 100,[107] a near perfect score on Chinese Music Charts while "To The Light" was rated 85.97.[107] I Will also performed well on the Gaon Chart, peaking 1st in physical album raking and 3rd on the Gaon Social Charts in the third week of March.[108] F.T. Island sold 31,000 copies of the album peaking in 7th position for the highest number of album copies sold in the entire month of March.[109] "Pray" was also a huge success in China as they were awarded the Most Popular Band in Asia, one of the three major awards in that event. The members could not attend the event, but gave a video recording to thank fans for receiving the award.[110] F.T. Island next tour was titled as We Will which was held in China, Japan, Taiwan and more.[111]

After their Korean comeback, F.T. Island then returned to the Japanese market with a new fully composed album, 5.....Go released on May 13. The title song of the album, "Primavera" is a collaboration with Japanese rock singer Takahiro Moriuchi from One Ok Rock.[112] The official M/V was released on YouTube on April 18. "Primavera" is a rock ballad song which means "spring" in Latin. In the album, it contained 10 fully self-composed songs where "Fish" was included in the album as an acoustic version. The album was also separated into 4 different sets. The first set contained only the songs whereas the second contained miscellaneous such as the official music video; the last two sets contained different performances for their Summer Sonic concert last year.[113] F.T. Island also celebrated their 5th Japanese anniversary with various tours and sang songs from their new album. It was a major career breakthrough as it received many good international praises and both the Korean and Japanese album brought a huge impact to viewers around the world.

3 months later, F.T. Island continued to release new songs titled "Puppy", which was a Japanese single and was included in the new album. "Puppy" was recorded to be F.T. Island's 35th single since their debut. In this new album, there are 2 new songs and it has 4 different sets. The first set contained only the songs whereas the second contained an additional DVD concert which was from 2014; the third set contained additional fan meeting footages while the last includes behind the scenes and other miscellaneous.[113] The official M/V was published on YouTube on August 16. The song was a new type of genre and it concentrated on the playful and humorous sides for each of the members. The music video was rather colourful and it was the first time where a foreigner was seen in F.T. Island's music videos. Upon release, the album immediately topped Japan's Music Chart.[114] The song also managed to claim the sixth position in Billboard's Japan Hot 100.[115] After "Puppy"'s success in Japan, F.T. Island then released the Korean version of the song.[116] The band then held an autumn tour in 13 different countries within 32 days. The tour consisted of Nagoya, Osaka and Sendai, and many more.[117]

In 2016, it was announced that F.T. Island would have Korean and Japanese comebacks this year as well. The Japanese studio album is titled N.W.U. It is the acronym of 'Naite' (泣いて, to cry), 'Waratte' (笑って, to laugh), 'Utatte' (歌って, to sing) and was released on 6 April.[118] All tracks in the album except the title song was composed and written by the members. The M/V for the song "You Don't Know Who I Am" was released on 18 March.[119] 7 days later, the highlight medley was released on YouTube.[120] Upon release, the album ranked first on the Japanese’s Tower Record, which is the largest record store in Japan, and second on the Oricon daily album chart. It also topped the HMV Asia daily charts and K-pop charts for famous Japanese sound sources such as Dwango.[121] An Arena Tour was also held, titled LAW of FTISLAND: NWU. The tour was held in Osaka, Tokyo and Aichi.[122] The tour was then extended to have another concert in Nippon Budokan Tokyo after gathering hot responses.[123]

On July 11, FNC Entertainment announced F.T. Island's sixth Korean studio album, Where's the Truth? which was released on 18 July. The title song is titled as "Take Me Now" and it is a track personally produced by the members.[124] The M/V was released on the same day in YouTube.[125] With the already success of their previous album I Will, Where's the Truth? managed to break its previous record by ranking in 5th position on Billboard's World Album Charts (I Will charted at 7th position). This was the band's first top 5 entry in this category and it was the best selling K-pop album in that week of its release.[126][127] The band still managed to maintain the top 10 position for the next week, at the ninth world's best selling album.[citation needed] The album also charted at 1st in the Gaon Album Chart for the week of July 23.[128] In an interview with the band, the band was asked why they wanted to pursue rock genre again, in which Hong-gi answered, "We threw away what is popular. We showed you popular rock for the past 8 years. In our 9th year, we started to slowly show you the music that we want to do. Lately, it’s hard to find band music on music charts. We wanted to show how we changed based on the fame we earned through popular music."[129]

On the same day as the release for the M/V for "Take Me Now", the band's 37th single "Just Do It" was also released.[130] The single was also a huge success as well. It topped HMV's Asian album chart, Amazon Japan's Asian pop chart; landed in the top 3 on Tower Records' world music chart and hit No.5 on the Oricon Singles Chart.[131] On September 2, Universal Studios Japan announced that F.T. Island is the first k-pop group to have a song that can be played in the ride "Hollywood Dream".[132][133][134] The song also came atop in Tower Records Japan for the top 10 k-pop singles and albums for the second week of September.[135]