The incident happened around 12:40 p.m. on Laurel Avenue in East Islip.
Authorities say the 22-month-old boy climbed the ladder of the above-ground pool.
When police arrived, the child's mother was administering CPR.
The boy was then rushed to Southside Hospital, where he was pronounced dead. He would have turned 2 in September.
The incident is being investigated as an accident.
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