Gaga, 25, started selling $5 (£3.10) We Pray For Japan wristbands in the wake of the catastrophe in March.
But lawyers at 1-800-LAW-FIRM based in Detroit, Michigan, filed a suit against the singer — real name Stefani Germanotta — hours before she headlined a gig in aid of Japan at the weekend.
They allege she inflated the costs of the wristbands and took the extra cash and refused to specify how much of the total cost actually went to the victims.
Alyson Oliver, who filed the suit at a federal court, added: “When you use your celebrity and your power as a musician to take money from people under false pretences, that's just wrong.
“When we tried to communicate with the defendants in this lawsuit, all we got was: 'Well, some of the money is being retained, but we don't really know how much.'
“If the money was going to the right place rather than into the defendant's pocket, then I hope that our effort helps that.”
The singer has yet to respond to the suit.