The France forward scored his team's first goal and was instrumental on the second as Marseille avoided a third defeat in five games this season.
The visitors went ahead after 15 minutes when the unmarked Daniel Niculae fired past Steve Mandanda with an angled shot.
Valbuena equalised three minutes before half-time with a stunning shot from just inside the box that went under Stephane Ruffier's bar following a poor clearance by the Monaco defence.
Park chu-Yuong made it two in the 79th minute minute at the end of a sharp counter-attack, only for Adriano to deflect a Valbuena shot into his own net two minutes later.
Marseille, in 13th place, still trail leaders Toulouse by seven points with unbeaten Monaco in ninth, five points off the pace.
Girondins Bordeaux's miserable start to the season continued with a 2-1 defeat at Nice.
The 2009 champions, who lost coach Laurent Blanc, playmaker Yoann Gourcuff and striker Marouane Chamakh during the transfer window, have picked up only four points from five games.
Nice's Gabon striker Eric Mouloungui was the first on target, 10 minutes before the interval, after he collected a through ball from Anthony Mounier to fire past Cedric Carrasso.
Habib Bamogo doubled the tally four minutes into stoppage time from a Didier Digard cross and Anthony Modeste scored a consolation for Bordeaux one minute later from the penalty spot.
Bordeaux were shaky in defence and toothless up front, with Modeste struggling to fill in for Chamakh, who has gone to Arsenal.
"Am I worried? Of course I am," Bordeaux coach Jean Tigana told reporters.
Promoted Stade Brest were held to a goalless home draw by neighbours Lorient.
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