Greyhound Bridge on the A589 will shut next January as part of a major refurbishment.
The bridge repair work will begin in January and is scheduled to be completed in June.
This will take around 23 weeks.
The bridge needs substantial maintenance to avoid the need for restrictions on the weight of vehicles it can carry.
Before the repairs can begin, the road network in the city centre will be remodelled to allow two-way running of vehicles on Skerton Bridge – as a temporary measure while the improvements are made.
These changes, along with other preparations for the repair scheme, will be put in place between September and December 2017.
Lancashire County Council has secured £3.7m from the Department for Transport to refurbish Lancaster’s Greyhound Bridge.
The funding was announced as part of a £75m government investment in 19 highway improvement projects across the country.
The total cost of the scheme will be £4.6m, with the county council providing additional funding of almost £1m for the project.
The funding will go towards improvements to ensure that HGVs and buses can continue to use the bridge, and help avoid serious disruption to the city’s transport system.
County Councillor Keith Iddon, cabinet member for highways and transport, said: “This is great news and a real boost for people who live and work in the Lancaster area.
“The Greyhound Bridge is a vital transport link for traffic travelling across the River Lune.
“We’ll now be able to begin a complete refurbishment of the bridge, which will mean we don’t have to place weight restrictions on it.
“Without this much-needed maintenance work, the only option would have been to to close the bridge to buses and lorries within the next two years and divert them through other areas of the city onto the Bay Gateway.
“This would create major disruption leading to longer journey times and increased pressure on other areas of the highway network.
“However the project will help us continue to improve traffic flows through Lancaster, and means we can achieve our aim of providing a transport network that supports the growth of the local economy and provision of skilled jobs and business opportunities in the area.”
David Hurford, bridges manager for Lancashire County Council, said: “We have known for some time that Greyhound Bridge needs a lot of work in order to avoid the need to impose weight limits in coming years.
“The extensive work means we will have to put some diversions in place and make temporary changes to the road network, but we’ll do all we can to keep disruption to a minimum.
“Detailed plans to prepare for this work have already been made, in case our bid was successful.
“We wanted to wait until the Bay Gateway opened before any work took place, as it will provide an alternative route, helping to reduce disruption while these repairs take place.
“We have consulted local businesses and residents about our proposed plans and will consider their responses when planning the changes to traffic management and the bridge repair work.
“We will provide more information nearer the time when the work is due to begin.”