The Vermont senator had 84 votes, topping Biden, who received 48 votes on Saturday in the U.S. territory in the Pacific Ocean. Because it is a territory and not a state, Northern Mariana Islands residents are not able to vote in the general election.
Sanders picked up four of the six delegates up for grabs, but Biden still holds a 860 to 706 lead to secure the Democratic nomination.
The two will face off Sunday night in the first head-to-head debate of the campaign cycle. The debate was originally supposed to take place in Phoenix, but was moved to Washington, D.C., over fears of the coronavirus.