The 18-year old midfielder has received his P-1 work permit and international transfer certificate and is now eligible to play for the Sounders.
Sounders coach Sigi Schmid was delighted the Colombia U20 international was able to finally complete his transfer to Seattle.
"Montaño is an exciting, young attacking player with a big upside," said Schmid. "He has pace and is two-footed. We look forward to his stay with Sounders FC."