In December 2011, the group held a three-day concert tour in Tokyo, Nagoya, and Osaka. By the end of the year, T-ara had become the second most popular girl group in Korea. Also in December, T-ara made a ballad with Davichi called "Uri Saranghaetjanha" (우리 사랑했잖아, "We Were in Love"). It was later included on T-ara's "Funky Town" re-release album. "Uri Saranghaetjanha" made it to number one on the "Gaon" chart and number two on the Billboard Korea K-Pop Hot 100. Both groups performed the song on SBS's "Inkigayo" television show.
T-ara re-released "Black Eyes" under the title "Funky Town" in January 2012. The album made it to number one on the "Gaon" weekly chart and number two on the monthly chart. "Lovey-Dovey" was the second single from the album. It made it to number one on both the "Gaon" and "Billboard Korea" charts.
In February 2012, Forbes Korea listed T-ara as number 17 in their list of "Top 40 Power Celebrities". In April, Core Contents Media announced that they would add two new members to T-ara. Areum joined T-ara in June 2012, bringing the group to eight members. The second new member was to be Dani, who was 14 years old at the time. While she starred in T-ara's "Day by Day" music video, she did not end up becoming a member.
T-ara made their first Japanese studio album in June 2012 with the title "Jewelry Box". It made it to number two on the "Oricon" weekly albums chart with sales of over 57,000 copies. Following the album release, T-ara held a concert tour in Japan.
In July 2012, T-ara released their fourth extended play (EP), with the title "Day by Day". It made it to number five on the "Gaon" chart. The title track, also called "Day by Day", was released as a single on the same day. It made it to number two on the "Gaon" chart. On July 7, T-ara performed "Day by Day" live on "Music Core" with a 70-person orchestra. This was the first time Areum performed with the group.
In late July 2012, T-ara held a concert in Japan. Hwayoung only performed one song due to a leg injury. Afterwards, one member of T-ara tweeted a message about showing determination. Later four of the other members agreed with her. After this Hwayoung and her twin sister (Hyoyoung) tweeted about a "bad person". This lead to rumors of conflict within the group. Some assumed that Hwayoung was being bullied. Soon after, Core Contents Media ended Hwayoung's contract and she left T-ara. The stated reason was that Hwayoung's actions were harming teamwork. The record label claimed that the bullying rumors were not a factor in the decision. The group stated that the rumors were not true but did confirm a conflict between Hwayoung and the others. Due to the rumors, the group put all activity on hold for a time. Their scheduled performances were postponed without a new date planned.
In September, T-ara resumed their activity as a group. In that month they released "Mirage", which was a re-issue of their "Day by Day" EP with additional songs. Two of the songs from the EP were also released as singles on the same day. These were "Sexy Love" and "Najgwa Bam (Love All)" (낮과 밤, "Day and Night"). "Sexy Love" made it to number four on the "Gaon" charts and number three on the Billboard Korea K-pop Hot 100 charts. While the new EP and song were a success, the group’s popularity suffered a drop due to the bullying rumors.
Later in September, T-ara made a Japanese version of "Day by Day". In October, T-ara released a greatest hits album in Japan. They stated that it was in celebration of one year since their Japanese debut. Also in October, T-ara made a Japanese version of "Sexy Love" and traveled to Japan to promote it.
In March 2013, T-ara released their sixth Japanese single, with the title "Bunny Style!" It was their first song made just for the Japanese market. It was released in ten different editions. Each of these included a different second song along with "Bunny Style!" Seven of them had a solo song by one of the T-ara members. Three of them had a song performed by a sub-group (a smaller group formed from members within T-ara). To promote the single, T-ara held special concerts in 10 Japanese cities.
In April, T-ara formed a sub-group with members Eunjung, Hyomin, Jiyeon, and Areum. It was given the name T-ara N4. The sub-group made the song "Jeon Won Diary".
In July 2013, T-ara performed at the Nippon Budokan to celebrate the release of their second Japanese album "Treasure Box". That album was announced for August 2013. At the concert, it was announced that Qri would be the new leader of T-ara. T-ara later released the music video "Painkiller" as a digital single. The song was made with T-ara, Davichi, See Ya, 5dolls and SPEED. Jiyeon appeared as the main character in the music video. Also in July, Core Contents Media stated that Areum would be leaving the group "towards the beginning of next year".
In August, T-ara released the music video "Bikini" featuring Davichi and Skull. T-ara then held a concert in Mongolia, along with Davichi, SPEED, and The SeeYa. In October, T-ara performed in Korea for the first time since the bullying rumors. This included their new song "Number 9" along with "Sexy Love". Four days later they released "Number Nine" as part of a mini-album with the title "Again".
In December, T-ara performed in Korea with the song "Do You Know Me". They also released a repackaged album titled "Again 1977". It contains the most popular songs from "Again". The group also released a ballad version of "Do You Know Me". Later in the month, T-ara released the Christmas song, "Hide & Seek". By the end of 2013, T-ara was ranked seventh in by number of top songs in the past decade.
T-ara planned to make China a focus in 2014, with five scheduled concerts. They held the first of these in January, in Chengdu. In February, T-ara held a concert in Phnom Penh, Cambodia, along with SPEED. In March, T-ara performed in Japan and released the singles "Lead The Way" and "LA'booN". They also made their third Japanese album and gave it the title "Gossip Girls".
In September 2014, T-ara made their sixth mini album with the title "And & End". They released it along with two music videos for the title track, "Sugar Free". The song was inspired by EDM and is part of the big room subgenre of electro house music. On September 24, a remix album was released called "EDM CLUB Sugar Free Edition". This included an English version of "Sugar Free". This was the first song T-ara recorded in English.
In October 13, T-ara signed a contract with Chinese record label Longzhen Culture Development. The contract was worth 5 billion KRW (~4.8 million USD). Then in November, T-ara made a cover version of the Chopstick Brothers' song "Little Apple". It is partly in Korean and partly in Chinese, and includes the Chopstick Brothers for the chorus. This music video had ́8 million views within the first two days.
In December, T-ara held a small concert in Seoul. This was their first concert in 2014 in Korea. Later in the month, T-ara closed out their year with a concert in Shanghai.
T-ara held a concert in Korea in early January of 2015. Soon after, they went to Vietnam to perform. On January 9, the group arrived at the airport, but had problems entering the country. So the concert had to be rescheduled. It later took place on October 10.
In February 2015, MBK Entertainment formed a project group known as TS. It included Eunjung and Soyeon as well as members of other groups. The TS group made a winter single, titled "Don't Forget Me." In March 2015, T-ara was announced as the main performer for Malaysia's Sultan Of Johor Coronation celebrations in Johor Bahru. The group appeared at the event along with Sistar, and The Black Eyed Peas' Taboo.
In June 2015, T-ara began a China tour, called the Great China Tour. The tour started with Nanjing, Beijing and Hefei. The group performed 22 songs. The next concert on their Great China Tour was held in Guangzhou on December 19.