An air and sea search was launched after reports of a lone swimmer off the coast of Dundee.
A rescue helicopter was scrambled to the Stannergate area after a member of the public raised the alarm just before 6pm on Monday.
Two lifeboat crews also joined the hunt and clothes believed to have belonged to the missing man were recovered from the water.
However, searchers were stood down at 11pm and it is thought that the “swimmer” may actually have been a buoy.
A Police Scotland spokesman said it was reported that a person had been spotted in the River Tay.
A force spokesman said: “Officers were alerted by members of the public to a man ‘swimming’ in the water between Dundee and Broughty Ferry in the Stannergate area at around 5.40pm.
“The man is described as being in his 40s, white, with a bald head. He was wearing a red T-shirt, a black jacket, dark trousers and Converse trainers.”
He added: “Searches are currently being carried out by coastguard and lifeboat to try and find the man in question, and the Police Scotland helicopter is also expected to attend the area to help the search.
“Officers say the man could indeed be safe and well, however there are obvious concerns for his well-being.”
Police urged the swimmer to get in touch and let them know he is safe and well.
A spokesman for the Broughty Ferry RNLI team said: “The Inshore lifeboat was at the scene of the initial sighting quickly and immediately commenced a systematic and thorough search of the area. It was soon joined by the second lifeboat. Following the search where no one was found the lifeboat crews then focussed their search around and under the three oil rigs currently berthed in Dundee Port.
“During the search an item of clothing was found that was thought may have belonged to the missing person. The crews continued to search and focussed on an extensive shore to shore search around the Tay road and rail bridges, before the inshore lifeboat returned to station to allow a team of relief volunteers to take over. Both lifeboats then searched a large area of the river up to Tayport.
“Following a reported sighting near Newport the lifeboats along with the Coastguard helicopter made for the location, however no one was found. It was believed that a mooring buoy had been mistaken for someone in the water.
“Both RNLI lifeboats were stood down and returned to station just before 11pm as no person had been found.”
Anyone with information about the man, or anyone who may have seen someone matching his description, is urged to contact the force on 101.