Police said emergency service crews were called to Eastbourne Pier at around 7.20pm on Sunday, May 1.
A Sussex Police spokesperson said, “The man was pulled from the water by the lifeboat crew and taken to hospital in a serious condition.”
An eye-witness said the coastguard rescue helicopter was also at the incident.
An RNLI spokesperson said, “Once on scene, the inshore lifeboat quickly located the casualty who was recovered onto the lifeboat and life-saving casualty care [was] carried out.
“The casualty was then taken ashore and passed into the care of waiting ambulance personnel and coastguard rescue helicopter paramedic.
“Our station doctor had also been transferred ashore from the all weather lifeboat.”