Terms of the deal were not disclosed.
Kirby, 33, spent the last three seasons (2000-2002) with the Raiders, but his '02 campaign was cut short after he fractured his right fibula and tibula in a game last October.
In fact, Tuesday was not only Kirby's first practice as a Buc, but it was also his first practice since suffering the leg injury.
"It was my first practice since October when I broke my leg," said Kirby. "Today was a good day.
"It's a lot different once you're out there actually playing, but today was a good sign."
Kirby spent two of his three seasons in Oakland playing under Bucs head coach Jon Gruden. The fact that Kirby is familiar with Gruden's offensive system should help him get up to speed.
"That's a big thing because I know the system, so it's not like I'm coming in here and I'm far behind everyone," said Kirby. "Some of the terminology has changed, but it's pretty much the same (offense)."
A 11th-year veteran, Kirby has played in 110 games and rushed for 2,875 yards, averaged 3.8 yards per carry and has scored 27 touchdowns. He's also hauled in 333 passes for 3,222 yards and scored 12 touchdowns.
Kirby is confident he'll work his way into the rotation with Michael Pittman, Mike Alstott, Travis Stephens and Aaron Stecker, and help improve Tampa Bay's 27th ranked rushing attack.
"There's a lot of running backs here and they're pretty good back there," said Kirby. "I just want to come in here and hopefully we can bring that committee up to par and get ourselves our there and work well together."
While Kirby could work his way into Tampa Bay's running-back-by-committee approach, he could also compete with Karl Williams and Jacquez Green for a job as a return specialist. Kirby has returned three kickoffs for touchdowns and has averaged 22.7 yards per return during his career.
"Definitely," Kirby said when asked if he could contribute on special teams as a return man. "I'll help this team any way I can, whether it's returning kicks, I don't mind doing that. It's something I've done in the past and something I've done the last couple of years. Anything is possible."
Pittman's recent arrest for domestic violence might have suggested the team signed Kirby in order to cut ties with Pittman, but the team said Kirby's signing is merely a coincidence since he was invited to One Buccaneer Place for a workout last week, which was days before Pittman's brush with the law last weekend.
Prior to his three-year stint with Oakland, Kirby played with the Cleveland Browns, the San Francisco 49ers and the Miami Dolphins.
Kirby entered the NFL in 1993 out of Virginia as a third-round draft pick of the Dolphins.