Ys IV: Mask of the Sun (イースIV -MASK OF THE SUN- Īsu Fō^?) is an action role-playing game developed for the Super Famicom, and is the fourth game in the Ys video game series. Ys series creators Nihon Falcom Corporation licensed its development to Tonkin House.

Mask of the Sun was released in 1993 and was one of two games released under the title of "Ys IV", the other being Hudson Soft's Ys IV: The Dawn of Ys for the PC Engine. The two games share the same basic story, but many changes were made in the Hudson-produced Dawn of Ys. Of the two, Mask of the Sun was the official Ys IV storyline before the release of Ys: Memories of Celceta. Originally a version of "Mask of the Sun" was also planned by Sega for Mega Drive and later Mega-CD under the Sega-Falcom joint development agreement, but this version was ultimately canceled.

In 2005, Taito Corporation developed a remake of Mask of the Sun for the PlayStation 2, titled Ys IV: Mask of the Sun - A New Theory.