The Birthday of Rei Ayanami is the twelfth music album released in the Neon Genesis Evangelion franchise. As the title indicates, its focus is Rei Ayanami, one of the three primary protagonists of Neon Genesis Evangelion. The album features background music and instrumental tracks related to Rei's appearances in the Neon Genesis Evangelion TV series and movies. It also contains vocal tracks by her voice actor, Megumi Hayashibara, for the "Rei Ayanami versions" of certain songs related to Neon Genesis Evangelion. The Birthday of Rei Ayanami was released on March 30, 2001 in Japan by King Records, Hayashibara's 34th birthday. Rei Ayanami's fictional birth date however, is unclear. The Birthday of Rei Ayanami has only been released in Japan.
It peaked at number 45 in the Oricon albums chart and made just 3 appearances.
The album cover features an illustration by Yoshiyuki Sadamoto, the character designer for the series. The CD itself depicts the front part of Rei's plug suit bearing the inscription 00, which refers to her designated Evangelion, Unit-00. The album was produced by Kouji Asano and Takashi Tokuhara.