The 'Girls Just Wanna Have Fun' singer made the error while appearing at the US Open in New York this weekend.
Lauper's performance came following a moment of silence in tribute to the victims of the September 11, 2001 terrorist attacks.
While singing the line, "O'er the ramparts, we watched were so gallantly streaming", Lauper instead sang, "O'er the ramparts, we watched as our flag was still streaming."
Earlier this year, Christina Aguilera made the same mistake while performing the national anthem at the Superbowl.
Watch Cyndi Lauper's performance of 'The Star-Spangled Banner' below: