Spokeswoman Jen Psaki said Tuesday that the U.S. welcomes the release of the two Americans, but said privacy concerns prevented her from disclosing any details about how they were freed.
U.S. officials earlier identified the two as the captain and chief engineer of the U.S.-flagged C-Retriever offshore supply vessel.
Nearly all foreigners kidnapped are released after ransoms are paid.
Nigeria's navy has rescued at least two hostages this year and reported killing several pirates in counterattacks to prevent ship hijackings.
