The Major League Soccer club announced Monday it has also completed Romero's transfer from Brazilian club Tombense FC. The Argentine midfielder played the last two seasons in Montreal on loan.
Terms of the contract were not announced.
After struggling in his first campaign in Montreal in 2013, Romero proved his worth with six goals and three assists in 29 matches to earn team MVP honours last season. He was the league leader in shots on goal percentage at 64 per cent (20 on target in 31 attempts).
"I was confident we would reach an agreement," coach Frank Klopas said in a statement. "Andres did great things in 2014 and we believe he will continue to help this team in the future."
The 25-year-old previously played in Brazil's first and second divisions as well as with Argentinos Junior and the Argentine under-20 squad.