"Reunion" is a pop song by the Japanese duo and idol unit ClariS, written by Kz. It was released as the unit's seventh single on April 17, 2013 by SME Records. The song was used as the opening theme to the 2013 anime series Ore no Imōto ga Konna ni Kawaii Wake ga Nai.. A music video was produced for "Reunion", directed by Jungo. The single peaked at No. 2 on Japan's weekly Oricon singles chart.

"Reunion" is a synthpop song with instrumentation from a synthesizer. It is set in common time and moves at a tempo of 135 beats per minute in the E-flat major key throughout the song. The introduction starts with the synthesized music accompanying ClariS' vocals, and uses a bridge to transition into the first verse, followed by the chorus. After another bridge, this pattern is repeated for the second verse and chorus featuring the same music with different lyrics. A break is used to transition into the third verse, immediately followed by the fourth verse in the chorus melody. The chorus is used as the outro, and an instrumental coda is used to close the song.

"Reunion" was released in a regular edition and two limited editions on April 17, 2013 as a CD by SME Records in Japan. One of the limited edition versions was packaged with Oreimo artwork and also contained a short version of "Reunion" instead of its instrumental version. The other limited edition version came bundled with a DVD containing the music video for "Reunion". The song peaked at No. 2 on Japan's weekly Oricon singles chart and charted for 12 weeks. "Reunion" debuted and peaked on the Billboard Japan Hot 100 at No. 3.

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