Valeri tallied 21 goals and 11 assists in 2017, becoming just the second player in MLS history to record at least 20 goals and 10 assists in the same season.
The Argentine reached another milestone during the campaign when he scored in in nine consecutive games from July 29 to Sept. 24, the best run in MLS history.
The 31-year-old also set a league record for the most goals scored by a midfielder in a single season, beating the previous mark (18) set by Preki in 1996.
Valeri's heroics helped Portland finish atop the Western Conference, winning the division on a tiebreaker over arch-rival Seattle.
The 31-year-old easily won the vote, earning just over 50 percent of the combined tally between MLS players, club management and the media.
Last season's MVP, New York City FC striker David Villa, came in second in the voting with 16 percent of the combined vote.
Valeri is the seventh South American to be named MLS MVP and the third from Argentina, joining Christian Gomez (2006) and Guillermo Barros Schelotto (2008).