At this time the man, who has been named as Signaller Tyler Pearson, of 2 Signal Regiment, has not been located.
North Yorkshire Police were alerted at 2.28am on Thursday 3 April 2014, after a member of the public reported seeing a man in the River Ouse near Ouse Bridge.
Officers attended the scene and with the assistance of the Fire and Rescue Service began a search of the river and the river banks.
Searches are continuing today in and around the River Ouse, using numerous police resources including the Regional Underwater Search Unit and other specialist search teams.
Superintendant Phil Cain, of York Safer Neighbourhood Command, said: It is believed that Tyler entered the river voluntarily during a night out with a group of friends but at this time we cannot confirm whether he has exited the water safely.
He was seen to cross Ouse Bridge with two other people and then enter the river near the Lowther public house on King’s Staith.
It is believed that he was attempting to swim across the river to Queen’s Staith.
Specially trained officers are currently supporting Tyler’s family at what is an extremely difficult time for them.
It is important that we establish the exact circumstances surrounding this incident and I am appealing to anyone who saw Tyler either enter or exit the river to get in touch with North Yorkshire Police as soon as possible.
Anyone who can help officers with their enquiries is urged to contact North Yorkshire Police on 101 – select option 1 – and pass information to the Force Control Room.
City of York Council are committed to raising awareness of river safety and preventing river related incidents.
Steve Waddington, Assistant Director of Housing and Community Safety at City of York Council, said: Our thoughts are with those affected by recent tragic events. We continue to work with agencies and voluntary organisations to raise awareness of the need to take care by the city’s rivers and are determined to prevent river-related incidents where we can.
As a partner of the River Ouse Safety Advisory Group, we work together to provide support and safety advice through the joint emergency plan which we maintain, develop and exercise.
We urge everyone who is out and about in the city centre to watch out for each other and stay together when heading home so that should an accident occur, there is someone to raise the alarm.