The injury will hold him out of Friday's game against the Memphis Grizzlies, and he will undergo further testing to determine a full recovery timeline.
"He's obviously a really important player for us and is a guy who we envisioned as, and has done a good job at, the starting two guard," Warriors coach Steve Kerr said. "It's a concern for sure. We'll know more over the next few days."
Melton suffered the sprain during Tuesday's game against the Dallas Mavericks, though it's unclear when it happened. Melton never left the game, and finished with 14 points, four assists and four rebounds in 26 minutes.
He had started the previous two games, and Kerr said he wanted to keep him in that starting group moving forward.