The child, whose exact age was not disclosed, was symptomatic but has already recovered, the city's health department says.
In total, Long Beach has documented 20 confirmed or probable cases of monkeypox.
"To date, no one has required hospitalization and all are either recovered or are isolating and recovering at home," the Long Beach health department said.
The case in the child was described as probable but additional testing will be performed by the CDC for confirmation.
There have been at least four cases of monkeypox documented in children across the United States, though it appears that number does not include the new Long Beach infection.
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The announcement of the case comes one day after Gov. Gavin Newsom declared a state of emergency in California to provide more state resources in fighting the virus.
So far California has administered 25,000 doses of the monkeypox vaccines and plans to acquire more in the coming days and weeks and will also amp up its testing in partnership with local laboratories.
The state also said that access to the antiviral prescription drug to treat monkeypox, tecovirimat (TPOXX), is still limited, but the treatment can now be administered at more than 30 facilities and providers across the state.