Toronto acquired Canada international De Rosario from the Houston Dynamo in exchange for defender Julius James and allocation money on Friday.
Houston coach Dominic Kinnear said the move "is in the best interest of everybody."
De Rosario was Houston's highest-paid player last season at $325,000.
James played 13 games during his rookie season in Toronto, earning $44,750. The 24-year-old won his first cap for Trinidad and Tobago in September.
"Julius is an aggressive, fast defender that fits really well in our style of play," Kinnear said.