The emergency was reported shortly after 9pm on Monday, when police received reports that a man had been spotted standing on the wrong side of the barriers of the A27 bridge of the river, in Arundel.
The call sparked a major response, with 16 firefighters from Littlehampton and Arundel, joining police, paramedics, lifeboat crews and coastguard teams.
Nick White, lifeboat operations manager at the RNLI in Littlehampton, said: “We do what we always do. We did a thorough search of anything either side of the river and along the river’s natural flow.”
The teams spent the best part of three hours searching the river from Arundel right the way through to Littlehampton in an effort to find the man.
However, they were unable to find any trace of the male and called of the search at about midnight.
The operation then resumed on Tuesday morning, with lifeboat crews again patrolling the Arun, checking bridges, pontoons, and river banks for about four hours, with police using an unmanned aerial drone in the sky above.
However, like the night previously, teams were unable to find any evidence of a person or body in the water, and called off the search for the final time at about 3pm.
Ray Pye, RNLI spokesman, said there had some confusion, with police initially being concerned about a missing man from Littlehampton who might have been the person reportedly on the bridge over the river on Monday.
However, this proved not to be the case.
Mr Pye added: “Police spoke to that man and took him out the equation.
“Of course there might not have been anyone in the river. The person who spotted him was going round a roundabout at the time. When he came back, the man was gone.”
He added it was promising that no-one had been found and said the event had been ‘confusing’.