Kratz, 35, appeared in four games this season and made one start for the Royals and was hitless in four at-bats with one RBI on a sacrifice fly. He also played 15 games at Triple-A Omaha, where he hit .214 (12-for-62) with seven runs, two doubles, four homers and 12 RBIs.
On June 11, the Royals opted to reinstate Kratz from the 15-day disabled list and designated him for assignment, deciding to keep Drew Butera as their backup catcher.
Now in his sixth year in the Major Leagues, Kratz makes Boston his fifth team, having also played for the Pirates (2010), Phillies (2011-13), Blue Jays (2014) and Royals (2014-15).
Jordan Wilson is an associate reporter for MLB.com.