The right-hander started 31 games last season for the Baltimore Orioles, going 4-16 with a 5.10 ERA. He has also pitched for Atlanta (1997-2002), Philadelphia (2003-04), Cleveland (2005) and Texas (2006-09). For his career, Millwood is 159-137 with a 4.11 ERA.
Colorado also placed reliever Matt Lindstrom on the 15-day disabled list with a nerve injury in his upper right arm, optioned infielder Ian Stewart to Triple-A Colorado Springs and purchased the contract of pitcher Edgar Gonzalez from the top minor league affiliate.
Lindstrom is 2-1 with a 2.89 ERA in 50 appearances for the Rockies this season, while Stewart batted just .156 in 48 contests.
Gonzalez went 6-6 with a 4.70 ERA in 22 combined starts with Triple-A Durham and Colorado Springs.