Fate/kaleid liner Prisma Illya: Vow in the Snow (Japanese: 劇場版Fate/kaleid liner プリズマ☆イリヤ 雪下の誓い, Hepburn: Gekijō-ban Fate/kaleid liner Purizuma ☆ Iriya Sekka no Chikai) (also known as Oath Under Snow) is a Japanese animated fantasy action film and a prequel to the manga Fate/kaleid liner Prisma Illya by Hiroshi Hiroyama, released on August 26, 2017. The film stars multiple voice actors including Noriaki Sugiyama, Kaori Nazuka, Rikiya Koyama, Natsuki Hanae and Noriko Shitaya for the main characters. The plot follows a teenager named Shirou Emiya who becomes involved in the mage conflict known as the Fifth Holy Grail War to protect his foster sister, Miyu Satsuki, from being used as a sacrifice by the Ainsworth clan.

The film was first announced on September 21, 2016 when the finale of the anime's fourth season, Fate/kaleid liner Prisma Illya 3rei aired. Shin Oonuma and Silver Link returned from the anime series to direct and produce the film, respectively. Singer ChouCho performed two theme songs that are played in the film's ending primarily centered around Shirou's actions and his bonding with Miyu.

The film grossed $1,118,410 in Japan while the home media releases were also one of the best selling Japanese DVDs and Blu-rays by 2018. Sentai Filmworks released the film's Blu-ray in English regions during July 2020. Critical reception to the film has been positive for its handling of an alternate take of Shirou and his battles in the Grail War while also highlighting the visuals produced in general.