The person spoke to The Associated Press on condition of anonymity because no details were released about the threat.
Princeton lost to Maryland 85-70 on Monday in the second round of the tournament at the Terrapins' arena, the Tigers' only defeat of the season.
On Saturday, President Obama attended Princeton's 80-70 victory over Wisconsin-Green Bay in the first round, inevitably drawing attention to his niece, freshman forward Leslie Robinson. Her father, Craig Robinson, is the brother of first lady Michelle Obama.
A phone message threatening Leslie Robinson was left at Maryland and the threat was taken seriously, according to the person who spoke to the AP.
The threat was first reported by USA Today.
After Monday's game, Princeton coach Courtney Banghart confirmed that she was aware of the threat before tipoff but Leslie Robinson was not. The player did not appear in either of Princeton's NCAA Tournament games.
"It's incredibly disappointing that the first question we get (after) being 31-1 (and) in the NCAA Tournament is about a freak," Banghart said.
"College Park police, I'm sure, had that under control. Leslie is safe. She's in my locker room. No one loves her like I do. She's an important part of our team," Banghart added. "Keep the freaks out of our gym."
Neither the president nor the first lady attended Monday's game, although Supreme Court Justices Sonia Sotomayor and Elena Kagan sat near Princeton's bench.