The Texas Rangers, Sosa's original team, signed him to a minor league deal worth $500,000 on January 30, 2007. This was the same contract that Sosa turned down the previous year from the Nationals. The contract included an invitation to spring training, where Sosa competed for a spot in the lineup with Nelson Cruz, Jason Botts, and other rookies/prospects. Sosa was successful during spring training and was added to the team's 25-man roster. He started the 2007 season as the Rangers' designated hitter and occasional right fielder. At the same time, the Chicago Cubs awarded Sosa's number 21 to new pitcher Jason Marquis, who coincidentally served up Sosa's 600th career home run. This caused some concern, due to Sosa's accomplishments with the Cubs, including his status as the Cubs' all-time home run leader.
On April 26, 2007, Sosa made history by hitting a home run in his 45th major league ballpark. He has also homered in The Ballpark at Disney's Wide World of Sports, near Orlando, Florida, a usually minor-league and Spring training park that hosted a regular season series between the Rangers and the Tampa Bay Devil Rays in May 2007, although he did not hit a homer at the two regular season games the Cubs played at the Tokyo Dome in 2000 vs. the Mets.
On June 20, 2007, Sosa hit a home run off of Jason Marquis during an inter-league game against the Chicago Cubs. Sosa became only the fifth man in history, following Babe Ruth, Willie Mays, Hank Aaron, and Barry Bonds, to hit 600 regular season home runs. The home run was the first one that Sosa had recorded against the Cubs, and as a result he has hit a home run against every active MLB team. Sosa is the Cubs' all-time home run leader, having hit 545 with that team.