The 30-year old Jamaica international heads to the Dynamo in exchange for defender Nathan Sturgis and allocation money.
"I have always like Omar," Dynamo head coach Dominic Kinnear said. "He is a good player with good pace. He is someone who knows the league well. We were in an unfortunate situation with Calen Carr's injury, but Omar is a player who can come in and help us stretch the field."
Cummings spent six years with the Rapids, scoring 39 goals and providing 27 assists in close to 150 appearances, and helped the Denver club to the 2010 MLS Cup.
Sturgis departs the Dynamo after joining in December 2011 from Toronto FC and made a total of eight appearances.