Police, paramedics and rescue specialists from across Tayside have flocked to Tay Street in Perth as a major search effort is underway.
The SFRS water rescue unit dropped into the water earlier this evening with a search crew on board.
Meanwhile, rescuers with flashlights probed the northern beached end of Moncrieffe Island and the riverbanks on either side.
Inflatable watercrafts coursed up and down the river between the three bridges and along the Willowgate Channel.
Bus stops and laybys along the busy Perth street are packed with special operations units and eyewitnesses reported the initial emergency services presence just before 6.30pm and were later joined by a search helicopter.
A police spokesperson said: “Police Scotland searching River Tay in relation to a male shouting for help heard in the vicinity of Perth Bridge (A85) between 5.45pm and 6pm today.
“Any persons who may have any information in relation to this are asked to contact Police on 101 and quote incident number 2787.”