Wells, who is eligible for free agency after the season, can make $270,000 in bonuses based on innings pitched. He gets $50,000 for 195 innings, $100,000 each for 200 and 205 innings and $25,000 for 210 innings.
The Pirates have no remaining arbitration-eligible players after also reaching agreements with left-hander Oliver Perez ($1.9 million), outfielder Craig Wilson ($3.3 million), right-hander Ryan Vogelsong ($555,000) and outfielder Jody Gerut ($875,000).
Wells was 8-18 with a 5.09 ERA last season, when he was the only NL pitcher to lose more than 16 games. He is a combined 13-25 the last two seasons after going 12-14 in 2002 and 10-9 in 2003, his first two seasons with Pittsburgh after being traded by the Chicago White Sox.
Despite Wells' poor season, the Pirates chose to bring him back because they concluded it would cost them more to sign a replacement starter on the open market who can pitch as many innings. Wells has averaged 179 innings since 2002.
Wells received the worst run support of any major-league starter (3.07 runs) last season as the Pirates scored three runs or fewer in 23 of his 33 starts and were shut out four times.