South Wales Fire and Rescue Service received a call at 9.37pm on Thursday, July 16 reporting two people in the water near Watkiss Way.
It was reported they were seen swimming alongside an upturned kayak.
A search was launched, with HM Coastguard called shortly after 11.30pm.
“At around 21.37 last night, South Wales Fire and Rescue Service received a call reporting two people in the water near Watkiss Way, Cardiff Bay. They had been seen swimming alongside an upturned kayak.
“A systematic search of the area was conducted by South Wales Fire and Rescue Service boats and crews using an underwater camera. The coastguard helicopter assisted the search using thermal imaging.
“A partially inflated kayak was found by crews, which had been pulled up on a rocky bank in the area near Cardiff Bay barrage.
“South Wales Fire and Rescue Service do not believe there are any casualties as a result of this incident.
“It is now believed the persons reported swimming had left the area prior to the arrival of the emergency services.”
The fire service said their involvement ended at 2am on Friday, July 17.
A spokesperson for HM Coastguard said: “At 11.39pm last night (Thursday, July 16), HM Coastguard received reports of two people potentially in the water from a kayak at Cardiff Bay.
“Penarth Coastguard Rescue Team and an HM Coastguard Search and Rescue Helicopter were sent to search, along with South Wales Fire and Rescue service.
“The search was stood down with nothing found, pending any new information.”
