Lifeguards discovered broken bottles, the remains of barbecue and damage to Whiterocks unit's radio aerial on Wednesday morning.
A large rock which had been thrown at the hut left a two inch hole in the unit which had also been covered with indecent graffiti.
The RNLI estimated that repairs will cost £800.
The vandalism follows two of the busiest days on Northern Ireland's beaches this summer.
RNLI Lifeguard Supervisor Karl O'Neill said the damage to the aerial mount had threatened vital VHF communications while the rock damage meant the unit was no longer watertight.
He said: "Our lifeguards rely on the aerial to communicate with each other when on patrol and to communicate with their colleagues in the Coastguard in the event of an emergency.
"Thankfully the damage has not rendered our communications off-service but should it have and should it have happened during the last two days which brought thousands of people to our beaches to enjoy the good weather, lives could have been put at risk.
"It is very disappointing for our lifeguards, who have been working hard to keep people safe, to turn up this morning after two busy days and see the unit they need to carry out their job has been so badly damaged, it really does dampen spirits.
"Our lifeguards' primary role at Whiterocks and on all lifeguarded beaches on the Causeway Coast is to make sure the beach can be enjoyed safely by the public. We want them to be able to continue to do that safely and with peace of mind," he added.
Vandals have caused hundreds of pounds worth of damage to an RNLI beach lifeguard hut in County Antrim.