The terms of the agreement have not been finalized, according to an article on MLB.com.
Slaten, a left-handed reliever, reportedly weighed multiple offers this winter before choosing the Pirates because they "provided him with the best fit and opportunity."
Slaten will be the 12th non-roster player to receive an invitation to spring training and will compete with four other left-handed relievers for a spot in the Pirates' bullpen in 2012.
The Washington Nationals made Slaten, a six-year veteran who also spent four seasons with the Arizona Diamondbacks, a free agent on Dec. 12, 2011 when they chose not to tender his contract. The decision came after the lefty sat out nearly three months of the 2011 season with left elbow ulnar neuritis.
The lefty posted a 4.41 earned-run average in 31 games and 16 1/3 innings last season, but is still an attractive option against left-handed batters out of the pen, limiting them to a .241 batting average and .304 on-base percentage in his six years.
Slaten owns a 7-8 career record, 3.60 ERA and 104 strikeouts in 137 2/3 innings pitched.