The deal is for $10 million, according to a source, who said Pena will receive $5 million in 2011, and $5 million in January 2012, which will count against the '11 payroll.
Pena had a star-crossed season in 2010, hitting 28 home runs with 84 RBIs but hitting a career-low .196 and striking out 158 times in 484 at-bats.
"It's a real good fit," Cubs manager Jim Hendry said at a news conference Wednesday. "We haven't had a lot of left-handed power. ... The Howards, the Teixeiras, the Adrian Gonzalezes, those people that put up power numbers at that position either stay where they're at, tied up, or sign huge, lucrative long-term deals."
Pena, who has a reputation as a good clubhouse presence, solves two problems for the Cubs: He gives them a left-handed power presence in the lineup next to Aramis Ramirez, and he is a Gold Glove-caliber first baseman.
According to a physical report, the 33-year-old Pena, who had plantar fasciitis last season, has recovered from the condition.
"I'm extremely confident. I don't tend to look back on my failures and dwell upon them. I just feel that I've had a lot of adversity throughout my career and a lot of difficulties," Pena said. "I think that all of those difficulties have made me stronger and a better player and a better person and a better man. So instead of dwelling upon that, I kind of embrace the fact that it was a difficult year and I should be a little bit more polished now."
Pena said he turned down multiyear offers in favor of Chicago.
"I love the city. My family loves the city. I think we have a pretty good chance at winning," he said. "We have a park; the history, what can I say? It's truly one of those teams that when you're a kid, you kind of dream about playing for. Like I said earlier, when I was 10 years old, I used to watch George Bell, Ryne Sandberg, [Shawon] Dunston, Andre Dawson. It was a regular thing at home."
Pena will be reunited with hitting coach Rudy Jaramillo, whom he worked with as a young player in the Texas Rangers organization.
Pena spent the past four seasons with Tampa Bay. He had a .227 average with 39 homers and 100 RBIs in 2009, when he was selected for the AL All-Star team. His first season with the club was his best, when he batted .282 with 46 homers and 121 RBIs in 2007. The following season, he helped the Rays reached the World Series for the first time.
Tampa Bay won the AL East this year but lost to Texas in a five-game, first-round series. The Rays appear to be cutting back on payroll. In addition to Pena's departure, setup man Joaquin Benoit agreed to a $16.5 million, three-year contract with Detroit, and Gold Glove outfielder Carl Crawford and closer Rafael Soriano appear set to sign elsewhere.
"It wasn't any secret from the end of the season on that one of our top and most important priorities was to fill our void at first base," Hendry said. "We were looking, certainly, to add someone from the left side, that not only was a quality offensive player with some power but also a good defender and a high-character young man. We have certainly landed that. We have filled all of the essentials that we were looking for with Carlos."
Bruce Levine covers the Cubs for ESPNChicago.com and ESPN 1000. Information from The Associated Press was used in this report.