Sanders secured over 75 percent of the 1706 votes cast in Spain, compared to just under 24 percent for Hillary Clinton, Democrats Abroad reported on Monday.
The Vermont senator also won with a clear majority worldwide at 69 percent of the vote.
Unlike the Republican Party, the Democrats have allowed the party's international branch to host a Global Primary since 2008, where expats can vote in-person with the group Democrats Abroad, which is given state-level recognition in the Democratic National Committee.
The group will send 13 delegates and eight super-delegates this year to the Democratic National Convention, which is more than the state of Wyoming's total of 18.
Sanders picked up nine pledged delegates in the global primary, while Clinton secures an additional four delegates.
"Democrats Abroad congratulates Senator Bernie Sanders for winning the Global Presidential Primary," wrote international chair of Democrats Abroad Katie Solon in a statement.
"We are proud of the unprecedented turnout. A diverse constituency of Democrats of all voting ages, living across the globe, brought their perspectives and priorities to our Party's nominating process. These enthusiastic voters will lead their fellow Americans living outside the US to vote from abroad in November."
An estimated 8.7 million Americans live abroad - more than the population of Austria. Both parties tout elections like that of comedian Al Franken to the US Senate in 2008 as showing the impact of absentee and expat votes: Franken beat out his Republican opponent with the help of absentee ballots.
This year, Democrats Abroad voting centres were were set up in a dozen cities at around 150 places worldwide, including in Madrid and Barcelona at the beginning of the month.
More than 34,000 people voted in the primary worldwide, far exceeding 2008's battle between Clinton and Barack Obama by 50 percent, according to Democrats Abroad.