Littlehampton’s lifeboat went out on Saturday after a 999 call about a man in the river near Ferry Wharf.
The stations D Class lifeboat Ray of Hope and volunteer crew launched at 7.44pm in windy, overcast weather conditions.
The lifeboat crew carried out an extensive search of the reported area along the River Arun.
Due to the ebbing tide, it was decided to extend the search further down the river towards the mouth of the harbour. Both searches proved inconclusive.
Following the searches, new information was provided by the UK Coastguard and police, the lifeboat was stood down as it was presumed that the man was safe ashore.
The lifeboat returned to the station at 8.05pm where it was refuelled and made ready for service.