The grim discovery was made by a passing motorist at around 1.45am on the carriageway near to Tabley Lane Bridge in Higher Bartle.
The motorway was closed in both directions for more than two hours between junctions one and three - Kirkham to Broughton - while emergency services dealt with the incident.
Diversions were put in place to get traffic around the scene.
Lancashire Police later revealed the body was that of a man aged 55 and appealed for information to help establish what happened..
Sgt Claire Pearson said: “We are trying to piece together the last movements of this man.
“If anyone saw someone at Tabley Lane Bridge this morning or if you have any information that could help with our investigation, please get in contact.”
The eastbound carriageway of the motorway was re-opened around 4am and traffic blocks on the westbound were lifted about an hour later.
Anyone with information can contact police on 101 quoting incident reference LC-20160407-0066.