2011-12: Continued Japanese popularity, Return to Korea and rising popularity

On April 20, 2011, F.T. Island released their seventh Japanese single "Satisfaction" with accompanying tracks, "Friends" and "I Want", in which Jong-hoon participated in the composition of both songs. The single debuted at number two on the Oricon weekly singles charts, becoming the band's highest charting single in the country.[47][48] Their title song "Satisfaction" was also selected to be the ending theme song of Fuji Television's anime Toriko.[49]

On May 18, F.T. Island released their first major debut album in Japan, entitled Five Treasure Island. The album features songs from their previous singles such as "Flower Rock", "Brand New Days", "So Today...", and "Satisfaction"; it also contains several new tracks, including the soundtracks for Japanese drama Muscle Girl!, in which member Hong-gi played the lead male role. The album ranked first at the Oricon album daily chart.[50]

In late May, F.T. Island returned to the South Korean music scene after almost a year, with a mini-album titled Return. The album was released on May 24. Title track was "Hello Hello", a fast-paced rock song. The album featured five songs in total. "I Confess" featured all of the members' vocals.[7]

F.T. Island with CNBLUE during their Stand Up concert at Nokia Theatre L.A. Live, March 9, 2012

After releasing a tragic music video for their title track “Severely”, F.T. Island released their fourth mini-album, entitled Grown-Up. Featuring five songs, it is composed of slow-tempo rock ballads. The album was released on January 31, 2012 and secured the No. 1 position on the Hanteo album chart for total album sales on both the daily and weekly charts, with title track "Severely" topping various online music charts.[51] The album was completely sold out ten days after its release, recording some 50,000 copies sold.[52] F.T. Island also achieved the "Triple Crown" recognition for "Severely" on JTBC's Music On Top. "Triple Crown" is a recognition for any artist who have won a music programme three times consecutively and the song would be removed from further trophies.[53]

On March 9, F.T. Island and fellow label-mates CNBLUE performed in America for the first time at Los Angeles Nokia Theater.[54]

In April, the band returned to Japan to promote their 7th Japanese single "Neverland", which was released on the 18th. “Neverland”, a fast tempo rock song, was chosen as the opening theme song of the animation Ozuma, a celebration of the 20th anniversary of Japanese broadcasting system WOWOW.

F.T. Island released their second stiudio Japanese album 20 [Twenty] on May 16, 2012.[55] It debuted at number four on the weekly Oricon albums chart, selling 41,726 copies in its first week,[56] earning the band their best first-week sales in Japan.[57]

The same year, F.T. Island released their fourth Korean studio album Five Treasure Box and its single "I Wish" on September 10.[58] Within the first week of its release, the album sold over 30,000 copies in South Korea.[59]