A man was reported to have jumped from the Norfolk Bridge on the A259 at Shoreham at 3.25pm on Monday (August 19).
Police, HM Coastguard and a Shoreham RNLI inshore lifeboat were deployed to the scene and joined by a National Police Air Service helicopter. As of 6.30pm there has been no trace of the man and police are continuing their search.
Chief Inspector Shane Baker said: “An extensive search to locate the man is continuing and we would like to thank the partner agencies for their assistance.
“The full circumstances surrounding the incident are unknown but we are assessing all lines of enquiry.
“We are urging anyone who saw a man jump into the water or saw someone on or around the bridge at this time to get in contact with us without delay.
“You could hold vital information to assist our investigation and we would like to hear from you.”
If anyone saw the man jump or saw someone on or around the bridge at that time, they are asked to report online or call 101 quoting serial 762 of 19/08. However, if you see or have seen anyone in the river, please dial 999 immediately.