Antti Niemi stopped Eric Fehr, Alex Ovechkin and Nicklas Backstrom in the shootout after making 35 saves in regulation and overtime. Marleau tallied after Joe Pavelski missed on San Jose’s first shot.
Ovechkin scored his league-leading 33rd goal at 12:44 of the second period after Tyler Kennedy had tallied for San Jose at 13:30 of the first.
The Sharks took the opener of a three-game road trip, improving to 17-1 against the Capitals in the teams’ last 18 meetings.
Rookie goaltender Philipp Grubauer, making his 12th start in 17 games, made 28 saves for Washington.
It was the Capitals’ second straight 2-1 shootout loss and they have lost six of their last eight.
The Capitals got their first power play with 1:15 left in regulation when Marleau was called for hooking, but Niemi made a pair of saves on Ovechkin in the final minute and San Jose killed off the remainder of the penalty in overtime. San Jose led 1-0 after a first period in which Washington went over 10 minutes without a shot on goal during one stretch.
The Capitals actually had an early 7-1 advantage in shot, but Niemi made several good stops, including a glove save on wrister by Backstrom in the slot during a 3-on-2.
San Jose broke on top at 13:30. Following a Washington turnover, Kennedy deflected a Jason Demers pass by Grubauer from high in the slot for his fourth goal of the season.
The Sharks came in having scored an NHL-best 51 first-period goals while allowing the fewest (22).
The Capitals controlled play for much of the second period and made it 1-1 at the 12:44 mark.
Ovechkin, the NHL goals leader, was held without a shot for the first 32 minutes but his first one found the net. He took a pass from Karl Alzer in the left circle and fired over Niemi’s shoulder from a tough angle.
With San Jose in the Washington zone late in the period, Brad Stuart poked the puck past Grubauer, but the replay showed time had expired.
NOTES: San Jose D Matt Irwin returned after missing two games with an upper-body injury. . Washington’s Aaron Volpatti and Mike Brown of the Sharks bloodied each other and the ice during a short, fierce fight midway through the first period. . Washington’s last nine home games have been decided by one goal, with six going to overtime.