The incident happened just after 10:30 a.m. at Lovers Point Beach.
Nearby surfers and paddleboarders were seen trying to help the struggling swimmer following the attack.
Police say the male swimmer suffered significant injuries to his stomach and leg from shark bites, and was rushed to Natividad Hospital by ambulance.
City officials closed the beach until Saturday, June 25, as a precaution.
The Monterey Fire Department deployed a drone, with the help of the Seaside Fire Department, to try and locate the shark, but so far, there have been no immediate sightings.
They will continue to monitor the area during the closure.
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