El-Hassan was driving northbound through Namsan Tunnel No. 3 at around noon when he rear-ended another vehicle, causing his own car to flip, according to a police report and medical staff at nearby Inje University Paik Hospital.
Hospital staff said the envoy was pronounced dead upon arrival at the hospital.
The scene of the accident is in close proximity to the Lebanese Embassy in Itaewon.
Two people in the second car suffered injuries and were taken to Seoul National University Hospital, police said.
Police are investigating the exact cause of the accident, including whether the ambassador was driving under the influence of alcohol.
A report on the cause of death of the envoy obtained by The Korea Herald indicates El-Hassan might have died from an existing health condition and not the external injuries he suffered as a result of the car accident. He was born in 1954.
Lebanese Embassy officials declined to comment on the matter.
In September, the diplomat underwent police questioning over a hit-and-run accident in the Itaewon.
El-Hassan was assigned as the top Lebanese envoy to Seoul in April 2013.
By Philip Iglauer (firstname.lastname@example.org)