Transport for London says that work will start on Monday 15 November and continue until the end of January 2022.
The work will see the installation of permanent barriers to protect people walking and cycling on both sides of the bridge, replacing the temporary barriers that have been in place since 2017.
During construction work, Westminster Bridge will be restricted to one lane in each direction for all traffic.
Following agreement with Lambeth and Westminster councils, work will be carried out between 7am and 11pm Monday to Saturday and between 8am and 6pm on Sundays except bank holidays.
There will also be overnight closures on Westminster Bridge.
Diversion routes will be in place via York Road, Waterloo Bridge, the Strand and Whitehall for northbound traffic and via Victoria Street, Vauxhall Bridge Road, Albert Embankment and Lambeth Palace Road for southbound traffic. TfL says that the diversion routes will be clearly signed during the overnight bridge closures.
Cyclists will continue to be able to cross Westminster Bridge during the works. Alternative options for crossing the Thames include segregated cycle lanes over Waterloo Bridge, Blackfriars Bridge and Vauxhall Bridge and the unsegregated cycle lane on Lambeth Bridge.
Nick Fairholme, TfL's Director of Project and Programme Delivery, said: "Safety is our top priority, and the installation of these permanent measures at Westminster Bridge will play an important role in improving the safety on the bridge for those walking and cycling at this busy central London location.
"There may be some disruption to travel during construction work and I'd like to ask people in the area who need to travel to plan ahead for their journeys, using alternative routes where necessary."