Shoreham RNLI were paged at 3:50 am on Saturday.
They deployed the all-weather lifeboat to seek a potentially endangered man believed to have ventured into the waters near Brighton Palace Pier.
They were joined by Coastguard Rescue Helicopter 175 and local coastguard teams, uniting to conduct an extensive four-hour search.
Despite their thorough search, they found no trace.
The man was later discovered safe on land.