The inaugural goal came in the 18th minute when Patrice Bernier weaved his way past the defense and found the Brazilian, who got a shot past goalkeeper Bryan Meredith to christen the revamped Saputo Stadium.
Work to expand the 4-year-old, soccer-only stadium from 13,000 to 20,000 was begun in 2010 after the announcement the Impact would joining the MLS starting this season.
The team has been playing its home games at nearby Olympic Stadium in the meantime.
Justin Mapp, rookie Andrew Wenger and Lamar Neagle also scored for Montreal (4-7-3).
The Sounders (7-4-3) got their only goal from Eddie Johnson.