The Giants promoted the right-hander from Double-A Richmond before Saturday's game against the Chicago Cubs.
"I want to prove to myself that I can still do it," said Nathan, who signed a minor league deal Aug. 19.
Nathan, 41, was a sixth-round pick of the Giants in 1995 and began his career as a starter before moving to the bullpen. But after struggling in the 2003 playoffs, Nathan was dealt along with left-hander Francisco Liriano to Minnesota for catcher A.J. Pierzynski.
It turned out to a one-sided deal, as Nathan became one of the top closers in the game. He has 377 career saves and was selected to six All-Star Games for Minnesota and Texas. But he has appeared in only four major league games the past two seasons after a second Tommy John surgery.
"Ultimate motivation is to win a ring and from there it was kind of nice to prove some people wrong," Nathan said. "Once I had the injury, I don't think it was 5 minutes after we found out I was going to have surgery that people were already saying, `So you're retiring, right?'"
Nathan picked jersey No. 74, having been born in 1974.
Manager Bruce Bochy says the playoff-contending Giants plan to use Nathan in the sixth and seventh innings.
"What's impressive is what he's accomplished, but he still has such passion for the game that here he is pitching at Double-A," Bochy said. "And now here he is, back with the Giants."
Nathan appeared in three games with the Cubs earlier this season, but was then released Aug. 6 amid a roster crunch.
"He had one really good game up in Milwaukee (on July 24) where we got a couple guys on and he struck out the side," Chicago manager Joe Maddon said. "His pitches got up to the low 90s (mph) with some really good break. It was just a lack of opportunity with us. I'm really happy that he's there."