Crews were out on exercise when they were alerted to reports of someone waist deep in the water across the causeway to Sully Island at around 10.45am.
They quickly arrived at the scene and discovered the individual was fishing during the incoming tide.
The crew conducted a quick welfare check with the fishman, then stood down and returned to their exercise.
The causeway at Sully Island is particularly dangerous, particularly when people try to cross during a flooding tide, and Penarth RNLI thanked members of the public who alerted them.
A spokesman for Penarth RNLI said: “While out on exercise this morning, at 1045 the coastguard tasked our D-class volunteer crew to reports of someone waist deep in the water part way across the causeway to Sully Island.
“With a fast running incoming tide, the crew quickly stopped their training exercise and headed to the scene.
“On arrival it quickly became apparent that the individual was fishing the incoming tide.
“Following a quick welfare check with the fisherman, the crew were stood down and returned to their exercise.
“This was a false alarm with good intent, and we are grateful to the members of the public who raised the alarm.
“The causeway at Sully Island is particularly hazardous and can create life threatening situations for anyone attempting to cross during a flooding tide.
“This shout was the first for our trainee crew Robin on the boat and Hannah as shore crew!
“Well done to both of them for this achievement.
“With the hot weather forecast this weekend, remember if you see someone in trouble at the coast, call 999 or 112 and ask for the Coastguard”.
Members of the public are reminded to call 999 or 112 and ask for the coastguard in an emergency.
