Summer Days (サマーデイズ, Summer Days?) is a non-stop animation adventure game developed by 0verflow (オーバーフロー). Based on the same formula as School Days, it uses full animated video to progress the story with the player being able to make (or not make) a choice. It was released on a double-layered DVD and required Windows Media 9 and DirectX 9. Summer Days is smaller than its predecessor, weighing at 5.92 GB on the DVD. It also does not have the violent endings that made School Days infamous.

Summer Days is an alternate universe prequel to the game School Days, set in the summer holidays just prior to the timeline of School Days. All the original characters are present, except it assumes that they all first meet during the summer holidays as opposed to during the first year of school. Although Kotonoha and Sekai, the heroines of the first game, are both present, the main heroine is Setsuna.

There are also more characters the player can end up with given the choices made in the game.

Summer Days has been plagued by the release of multiple patches, some over 1 GB in size, to correct a production mistake. As a result, Japanese fans have rated this game one of the worst to have been released.