A spokesman said all lines were blocked due to the incident, which happened last night (September 7).
In a statement yesterday evening, he added: “It is with great sadness that we report a person has been hit by a train between Brighton and Worthing.
“All lines are blocked between these stations whilst the emergency services work to deal with this incident.
“Tickets for today will be honoured on services tomorrow if you have been unable to complete your journey.”
The railway line has now reopened after the incident.
The Southern spokesman added: “We work closely with @samaritans and support the work they do. If you or someone close to you has been affected by this or any incident, there is always someone to talk to.
“Contact Samaritans on 116 123 free at any time, on any phone. Help & advice is only a phone call away.
“If your journey has been delayed by 15 mins or more, you can claim Delay Repay compensation which you can choose to donate to @samaritans to support their life-saving work.”
British Transport Police have been approached for comment.