Swimmers were evacuated from the water at a Bournemouth beach after a “large marine animal” was spotted in the water.
The RNLI said it received reports of “significant movement” in the water at Boscombe, leading its lifeguards to take action on Wednesday.
The alert at the Dorset beach ended about 30 minutes later.
Steve Lutwyche posted on Twitter: “This is not a drill, I’m on the beach at #Boscombe where the RNLI have had to red flag the beach due to reported sightings of a shark!
“(Actually just reopened now. Where is Chief Brody when you need him!?)
A RNLI spokeswoman said: “On Wednesday 4 August, RNLI lifeguards received reports of a large marine animal in the sea near Boscombe Beach, Dorset.
“As a precautionary measure, lifeguards asked beach visitors to evacuate the water and put up red flags.
“Lifeguards used their Rescue Watercraft (RWC) to scan the area and visitors were able to go back into the water half an hour later.
“The RNLI recommends you always visit a lifeguarded beach and report any concerning wildlife to the lifeguards as soon as it’s spotted.”