As 13,000 runners plan to take to the city's streets, road closures mean there will be significant travel disruption.
Brighton Marathon takes place on Sunday, April 7, and with a route of more than 26 miles planned on the streets, many roads will be closed.
Preston Park railway station, near the start of Brighton Marathon route, will be closed the day until 11am.
Govia Thameslink Railway, the operator in charge of Southern and Thameslink services, urged passengers to allow extra time at Brighton station.
A spokesman for GTR said: "Trains will be a lot busier than usual.
"A queuing system may be in place at Brighton station so please allow plenty of extra time."
There will also be significant disruption to bus services across the city.
Brighton and Hove Buses is running a limited service across its network until 1.30pm when usual services will start to be restored.
Route 1 from Mile Oak to Whitehawk will operate in both directions via Portland Road, Portslade Station and Victoria Road instead of via New Church Road and the A259 for most of the day.
Route 6 from Down's Park Sainbury's to Brighton Station will operate in both directions via Portland Road instead of New Church Road.
Full details of Brighton and Hove Buses disruption can be found on its website: https://www.buses.co.uk/brighton-marathon-weekend.
The Regency route buses 28, 29 and 29A from Tunbridge Wells, Ringmer and Lewes will terminate at from Lewes Road Bus Garage until 1pm.
Normal service from Churchill Square will resume with the 1.51pm departure.
Normal service from the North will be the 12.02pm departure from Tunbridge Wells.
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