Network Rail will start work on the bridge at Burnaby Road today, which is 142-years-old and no longer strong enough.
Here’s everything you need to know about the closure.
How long will the station be closed for?
The railway firm wants to replace the bridge by the end of Friday.
Last week people walking past could see the new bridge, waiting to be installed, on the opposite side of the road.
What roads will be shut?
Burnaby Road and Park Road will be shut while the work takes place. It will allow access for heavy machinery and equipment.
Diversions will be in place.
What about train passengers?
Buses will replace trains between Portsmouth Harbour and Portsmouth and Southsea stations while the new crossing line is being put in place.
People who are travelling on towards the Isle of Wight are also being advised to allow extra time for their journey.
The replacement buses will run every 10 minutes from 6am to 11.20pm during the closure.
The station is due to reopen at 6am on Saturday morning.