Police said that the body of a man was found underwater near the motorway.
The M1 was closed in both directions between junctions five and six near Watford at around 15:00 GMT on Monday.
Officers said the death was being treated as “unexplained” and drivers were facing severe delays during the evening rush hour.
The southbound section reopened at around 18:30, the northbound section reopened after 22:00.
One of three northbound lanes remained closed Tuesday.
A spokesman for Hertfordshire Police said: “We were contacted earlier this afternoon after a man’s body was found on the side of the motorway between junctions five and six.
“Officers are currently working to establish the circumstances of the man’s death, which are currently unexplained, and will remain in the area this evening and tomorrow.”