According to early results on a government website, statehood drew 97% of support with more than 90% of votes counted Sunday afternoon, but less than one in four voters participated in the plebiscite as opponents called for a boycott. Polls closed at 3 p.m. Sunday.

The vote was spearheaded by the territory's governor, Ricardo Rosselló, who has pushed for statehood as a way to help improve the island's economy, which is weighed down by debts of more than $73 billion. In May, Mr. Rosselló declared what amounts to the largest-ever municipal bankruptcy in the U.S. that placed Puerto Rico under court protection.

Congress would need to authorize a new state. Mr. Rosselló recently signed into law a measure creating a commission to press U.S. lawmakers for admission. On Sunday evening, he said he would visit Washington, D.C. to formally notify Congress and the White House of the results.