"Kitto Eien ni" is pop singer Crystal Kay's nineteenth single.

It was released on January 17, 2007 as the first single for Kay's seventh studio album All Yours.

The song, a ballad, was used as the ending theme of the live-action film adaptation of the manga Boku wa Imōto ni Koi o Suru (僕は妹に恋をする, lit.

I Love My Younger Sister) by Kotomi Aoki which stars Jun Matsumoto and Nana Eikura as Yori and Iku, fraternal twins who fall in love despite being siblings.

The single's B-side "As One" was used as J-Wave's Christmas campaign song in December 2008.