The alarm was raised just after 11am on Friday by a fisherman who rang 999 to say his friend had fallen overboard after their vessel had broken down a mile off Tunstall in Holderness.
He tried to help by throwing a lifering into the sea and also deploying a flare.
Humberside Coastguard Rescue Helicopter was scrambled alongside Withernsea and Humber RNLI Lifeboats, Withernsea and Hornsea Coastguard Rescue Teams and Hornsea Inshore Rescue Boat.
A number of nearby fishing vessels are also searching the area after responding to a Mayday Relay broadcast by HM Coastguard.
The helicopter has now stood down but the other vessels are continuing to search.
Kaimes Beasley, Duty Controller for HM Coastguard said: “This is an extensive search that is still ongoing. We have a number of rescue assets and local vessels assisting in the search operation and the search conditions are favourable.”