Georgetown University graduate students voted by a margin of 555 to 108 to unionize, becoming the latest in a growing list of campuses with organized students.

The nearly 1,100 graduate students, located in Washington, will be affiliated with the American Federation of Teachers. The vote took place Nov. 5 through Nov. 8 and ballots were tallied Nov. 9.

Voting results exceeded expectations, the Georgetown Alliance of Graduate Employees, the group behind the unionization bid, said.

“I am absolutely thrilled so...