Officers are working with partner agencies to search the river and the surrounding banks in York city centre in an attempt to locate the man.
There will be significant activity around Ouse Bridge as efforts to find the man continue and it is unknown how long the search will take.
Certain parts of King’s Staith and Queen’s Staith will be closed to members of the public while the enquiries are ongoing.
There are no further details of the man available at this time.
Anyone who may have witnessed the incident or who has any information which can assist the investigation is urged to contact North Yorkshire Police on 101 – select option 1 – and pass information to the Force Control Room.