The party says 34,570 US citizens living abroad in 38 countries cast votes by Internet, mail and in person from March 1-8. Sanders received 69 percent of the vote to earn nine of the 13 delegates at stake. Hillary Clinton won 31 percent, picking up four delegates.
It's an important victory for Sanders, who was swept by Clinton in five states last Tuesday.
Still, Sanders continues to trail Clinton by more than 300 delegates.
Clinton now has 1,163 delegates to Sanders' 844, based on primaries and caucuses.
When including superdelegates, or party leaders who can support any candidate, her lead is even bigger 1,630 to Sanders' 870. It takes 2,383 delegates to win the nomination.
Halfway into the primary season, the Democratic race now moves to Western states this week that Sanders is counting on winning to cut into Clinton's lead. On Tuesday, Democrats vote in Arizona, Idaho and Utah, with 131 delegates up for grabs; on Saturday, Alaska, Hawaii and Washington hold caucuses with 142 delegates at stake.