A 23-year-old man from Storrington has been arrested on suspicion of causing grievous bodily harm.
A man in his twenties received a small stab wound to his body at around 4am.
He was taken to hospital where he remains.
His injuries are not currently believed to be serious.
Detective Sergeant Colin O’Hare, of West Sussex CID, said: “As part of the investigation into this incident it was necessary for us to close Shoreham footbridge so officers could search it for evidence.
“This is a busy thoroughfare and the closure would have inconvenienced many people.
“I’d like to thank the community in Shoreham for their patience and understanding while the bridge was closed.”
The bridge has now reopened.
Anyone with information regarding the incident is asked to contact Sussex Police quoting serial 0105 of 10/1 or the independent charity Crimestoppers anonymously on 0800 555 111.