Birthday (バースデイ, Bāsudei) is a story collection by Japanese writer Koji Suzuki and first published on February 5, 1999 in Japan.

The book consists of three short stories occurring in different timeframes within the Ring universe: In November 1990 during the events of Spiral, Mai Takano wakes up at the bottom of an exhaust shaft of a building near Tokyo Bay.

As a former aide of Ryuji Takayama, who died of the ring virus, she was ordered by his publisher to search for some missing work papers in his childhood home.

The baby Sadako cuts her umbilical cord from the placenta, wipes herself with the towel, and leaves the shaft using the cloth.

Yoshino has just attended Kazuyuki Asakawa's funeral and also receives news of Mai Takano's death in an exhaust shaft.

Toyama is nearing his fifties, a twice-married man with children and a stable job, but he longs to meet Sadako, the only woman he truly loves.

He recounts to Yoshino events that transpired 21 years earlier, when Toyama was merely a young sound director trainee of Theater Group Soaring.

Toyama had a secret affair with Sadako, who begged him not to reveal it to outsiders as she still wanted to achieve success as a stage actress without controversy.

Amano reveals to Reiko about the LOOP project, a simulated but alive universe mirroring the real world, how it was frozen 20 years ago just when the project was consumed by Sadako and her ring virus/Metastatic Human Cancer Virus (MHCV), and how Kaoru Futami, a reincarnation of Ryuji Takayama and Reiko's lover, had found a cure to neutralize the virus.

Reiko dons virtual reality goggles and gloves and is thrust into the LOOP, where she meets Kaoru attempting to assure her that everything is alright.

She periodically checks the LOOP to see Kaoru, whom she learns has rapidly aged from a 20-year-old to a 37-year-old, and finally to an old man in his sixties.