Karstens can earn an additional $1 million in performance bonuses based on innings.
The 30-year-old right-hander was 5-4 with a 3.97 ERA in 15 starts and four relief appearances last year, when he missed time because of separate injuries to a hip, groin and shoulder. He made $3.1 million.
Karstens gives the club insurance after free agent left-hander Francisco Liriano injured his right arm last month, a deal that has led to a two-year contract not being finalized.
Karstens will be joined in spring training by two of the team's most promising pitchers in former first-round picks Gerrit Cole and Jameson Taillon. Both received invitations to join the team in Bradenton next month to spend some time with the big club.
Cole, the top overall pick in the 2011 first-year player draft, went 9-7 with a 2.80 ERA in 26 minor league starts last year. The 22-year-old right-hander rocketed through the team's farm system in 2012, beginning the season in Class A and ending it in Triple-A Indianapolis.
Taillon, the second overall pick in the 2010 draft, went 9-8 with a 3.55 ERA and 116 strikeouts in 26 combined starts between Class-A Bradenton and Double-A Altoona in 2012.
The Pirates also signed veteran outfielder/first baseman Brad Hawpe to a minor league contract. The 32-year-old Hawpe has a career .276 batting average with 124 home runs and 490 RBIs in 893 career games. He last played in the majors with San Diego in 2011, when he hit .231 with four home runs and 19 RBIs.