A rescue mission has been launched after a man was seen to walk into the sea off Blackhall Rocks.
An angler, who was among several on the beach, called 999 after seeing the man take off his coat and head into the water.
He has not been seen since,
His coat has been recovered, but his identity has not yet been established and it is not thought any reports of missing men match his description.
The alert was made just after 10am today, leading the HM Coastguard's officers from Hartlepool and Seaham, Hartlepool's RNLI teams and Durham Constabulary to launch a search.
The police helicopter has been drafted in as part of the effort.
A spokesman for the force said: "Emergency services, including police, Coastguard and air support have launched a search next to Blackhall Rocks after they received a report of a man in difficulties in the water.
"The man has not been seen since 10.15am and the search continues."
HM Humber Coastguard requested the launch of Hartlepool RNLI’s Atlantic 85, Solihull, Inshore lifeboat at 10.03am.
The inshore lifeboat was launched at 10.08am and was on scene at 10.16am.
The RNLI said when nothing was immediately obvious, HM Humber Coastguard then requested the launch of Hartlepool RNLI’s Trent class All weather lifeboat, Betty Huntbatch to assist in the search at 10.33am.
The All weather lifeboat was launched at 10.40am and on scene at 10.50am.
Both lifeboats began search patterns along the shore line from Dene Mouth to Horden Point, 2 miles to the north and 1.5 miles out to sea with the assistance of Coastguard mobile units and the police helicopter.
Hartlepool RNLI coxwain, Robbie Maiden said: "We carried out a number of search patterns in conjunction with the Inshore lifeboat at the request of HM Coastguard over several hours but unfortunately, even though the sea state was almost flat calm and the visibility was good, nothing was found."
Mike Craddy, Hartlepool RNLI operations manager, said: "The crews were stood down by HM Humber Coastguard at 3.40pm after over 5 hours thorough searching which proved inconclusive and it is uncertain at the moment whether the search will resume in the morning."
Anyone with information can contact police on 101.