The man was walking with his wife when he saw the 15-year-old flailing close to the Chichester Harbour entrance off Sandy Point at about 8.30pm on Friday.
He jumped into the water and brought the boy ashore, where he was reunited with a pal, also 15, who swam to shore after struggling himself.
The pair were recovered from the beach by the RNLI and taken back to the Hayling Island Lifeboat Station for treatment.
Alan Bartlett, the station’s volunteer lifeboat press officer, said: ‘A brave, concerned individual risked their own safety by going to rescue the young boy.
‘Whilst we don’t advocate that, we recognise that this courageous act resulted in the boy ultimately being brought ashore safely.’
The boys’ medical condition was checked by the station paramedic and the shore crew who agreed an ambulance was not needed.
Both swimmers and an adult relative were educated on the dangers of swimming in the harbour mouth.
The boys are believed to be from Hayling Island.
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