Roby, 24, is 6-foot-8 and was recently waived by the Thunder. He had 48 hours to clear waivers before becoming an unrestricted free agent. But San Antonio nabbed him before that process was complete.
Roby spent three seasons with the Thunder, averaging a career-high 10.1 points to go with 4.8 rebounds last season, appearing in 45 games.
Originally drafted by the Pistons in the second round in 2018 (No. 45 overall), he has also spent time in the G League with the Texas Legends and OKC Blue. He was waived by the Thunder on July 3.