The appointed agency will devise Southern's promotional strategy for the next two years. It will be responsible for co-ordinating promotions and advertising across TV, radio and the regional press in Sussex, Surrey and the South Coast.
The company rebranded as Southern last year. Its franchise is held by Govia, a joint venture between UK group Go-Ahead and French operator Keolis.
Southern is looking to shortlist up to five agencies, which will be invited to pitch early in the new year.
Govia won an eight-year franchise to operate across Kent and parts of Sussex at the end of last month. The services were previously operated by South Eastern Trains. It is not known whether the contract will cover its new franchise area. South Eastern Trains' advertising was handled by Rapier.