Sasha, using her full name, Natasha, has been accompanied to the restaurant by a contingent of six secret service agents, the Boston Herald reports.
The town has been a favourite location for the Obamas on their summer breaks.
Photographs show the president's youngest daughter wearing the restaurant's uniform of blue T-shirt and cap and working at a till.
A co-worker told the Herald: "She's been working downstairs at takeout.
We were wondering why there were six people helping this girl, but then we found out who it was."
Too funny. Poor Mexican got beat out of a job I guess.
US First Daughter Sasha Obama serves seafood in summer job - BBC News