Police were called around 9.30am following reports a man had fallen from a bridge above the M61 close to Junction 8 at Chorley.
Emergency services attended and a 76-year-old man from Chorley was found with serious injuries.
He was pronounced dead at the scene by paramedics.
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The death is not being treated as suspicious and a file will be passed to the coroner in due course.
Sgt Dave Hurst, of Lancashire Police, said: “These are tragic circumstances and my thoughts are with the man who has died, as well as his family and friends, at this difficult time.
“We would encourage any motorists, travelling in the area around the time of the incident, to come forward if they have dashboard mounted camera footage which can assist our investigation.
“Anyone with information can contact police on 101 or email [email protected]”