.hack//Sign (stylized as .hack//SIGN) is a Japanese anime television series directed by Kōichi Mashimo, and produced by studio Bee Train and Bandai Visual, that makes up one of the four original storylines for the .hack franchise. Twenty-six original episodes aired in 2002 on television and three additional bonus ones were released on DVD as original video animation. The series features each characters designed by Yoshiyuki Sadamoto, and written by Kazunori Itō. The score was composed by Yuki Kajiura, marking her second collaboration with Mashimo.

The series is influenced by psychological and sociological subjects, such as anxiety, escapism and interpersonal relationships. The series focuses on a Wavemaster (magic user) named Tsukasa, a player character in a virtual reality massively multiplayer online role-playing game called The World. He wakes up to find himself in a dungeon in The World, but he suffers from short-term memory loss as he wonders where he is and how he got there. The situation gets worse when he discovers he is trapped in the game and cannot log out. From then on, along with other players, Tsukasa embarks on a quest to find the truth behind his abnormal situation.

The series premiered in Japan on TV Tokyo from April 4 to September 25, 2002. It was broadcast across East Asia, Southeast Asia, South Asia and Latin America, by the anime television network, Animax, and across the United States, Nigeria, Canada and United Kingdom, by Cartoon Network, YTV and AnimeCentral (English and Japanese) respectively. It was distributed across North America by Bandai.

The storyline moves at a leisurely pace, and has multiple layers—the viewer is often fed false information and red herrings, potentially leading to confusion until the true nature of events is unveiled towards the end of the series. It relies on character development and has few action scenes; most of the time character interaction is presented in the form of dialogue. English language reception to the series has been generally positive, but some of these sources have negatively criticised the series as a result of its slow pacing and character-driven storyline.