Miller's agent, Andy Miller, confirmed the news to the San Antonio Express-News.
The addition of Miller provides veteran insurance at the guard position, where the Spurs are currently without top backup Manu Ginobili, who is still recovering from testicular surgery. Spurs coach Gregg Popovich provided an update on Ginobili's condition before Saturday's 104-94 win over the Houston Rockets, saying the veteran is coming along "slowly, but surely," and displaying "a little bit more movement each day."
"[There's] a little less pain each day," Popovich added, "but [it's] a slow process."
Popovich declined to delve into specifics regarding Miller, but considering San Antonio's roster is at capacity, the Spurs will have to release a player to accommodate him. Guard Ray McCallum, who has had multiple stints this season in the D-League, would appear to be the most likely candidate.
"I've been doing this a long time," Popovich said. "I've never talked about who we may or may not sign at any point in the season. We just tell you guys and gals whenever we do something."
Playing in his 17th season, Miller will don the uniform of the ninth franchise of his career. He averaged 3.4 points and 2.2 assists in 10.8 minutes in 26 games for Minnesota prior to the buyout.
Sources said Saturday that the Spurs have made the strongest pitch among the various playoff contenders chasing Martin.
ESPN NBA Senior Writer Marc Stein and ESPN Spurs reporter Michael C. Wright contributed to this report.