The incident took place between the Seaburn and Stadium of Light stations at 10.45pm.
Services were suspended between Pelaw and South Hylton while emergency services dealt with the incident.
A Nexus spokesman said: "A person was struck by a south bound Metro train at around 10.45pm on Saturday night. This has resulted in a fatality.
"Metro services were immediately halted and the emergency services were called to scene.
"All of our major incident procedures were followed. An investigation is now underway to establish what happened."
One passer-by said: "We saw the emergency services walking along the tracks towards a stationary Metro.
"This section of wall also has gated access to the train lines and was right where the Metro had stopped.
"Our attention was immediately drawn to all the passengers on the Metro looking out the window at the emergency services who had the track lit up with emergency lights.
"We were literally six feet maximum from where the paramedics were trying to get access under the stationary Metro.
"After a while the train pulled forward about eight feet and we could clearly see a body lying under the Metro."