Emergency services closed the M27 westbound at junction 9, by Whiteley, after the incident yesterday morning.
The man, believed to be in his 40s, plummeted more than 20ft on to the road, landing on the hard shoulder – narrowly avoiding traffic.
Witnesses to the horrifying fall sprinted to his aid, with some leaping over barriers to access the motorway and provide help.
Within minutes paramedics were on the scene treating the injured male.
Charlie Redding witnessed the whole ordeal.
The 17-year-old, of Whiteley, was walking on the bridge towards Swanwick train station when he spotted a man climbing over the railing.
‘He was carrying these shopping bags and he just put them down, climbed over the railing and stood on the edge of the bridge,’ he said.
‘I couldn’t believe what I was seeing. It was surreal.’
The teenager then saw the man fall from the bridge, landing on the concrete below.
‘Everyone was just in shock,’ he said. ‘Everyone went silent.
‘I thought he had died. It was a high bridge.
‘I ran over to check on him and thought in my mind he was gone.
‘He was lying on the ground twitching.
‘A few people ran on to the motorway.
‘Two or three people were comforting him.
‘It was really horrible.’
Emergency services were alerted to the fall at 10am.
Police closed the road while paramedics began treating the man, with an ambulance and rapid response vehicle in attendance.
The Hampshire and Isle of Wight air ambulance was called and landed on the highway but did not take the man to hospital.
A spokesman for South Central Ambulance Service said: ‘We received a number of emergency 999 calls just before 10am reporting a male having fallen from the bridge on to the carriageway at junction 9 of the M27 westbound.
‘Fortunately he was not hit by any vehicles travelling on the motorway.’
The man suffered back, pelvic and leg injuries.
He was taken to Southampton General Hospital by ambulance.
His injuries were not life-threatening.