People using buses in the West End of London will have to buy a ticket before boarding the vehicle from Saturday.
London's buses carry 5.4m passengers every week day
More than 300 ticket machines at all the stops in the West End will allow people to buy single tickets or one-day passes before getting on the bus.
It is part of Transport for London's (TfL) campaign to make buses cashless, partly to improve punctuality and partly to make drivers' jobs safer.
TfL said the majority of passengers already use travel passes and ticket books and only 16% pay cash in central London.
A campaign to inform bus passengers of the change is being launched by TfL's Peter Hendy and Toyah Willcox, who is starring in the West End in Calamity Jane.
More than a million leaflets will be handed out and there will be posters at bus stops and on buses.
TfL said the area covered by "pay before you board" includes 298 bus stops and is served by about 60 routes.
Peter Hendy said the ticket machines will enable passengers to buy £1 single journey tickets or £2 one-day passes.
"Buses have benefited from less traffic in central London, due to the success of the congestion charge", he said.
"But there's still more we can do to make the service even more reliable.
"This new initiative will reduce the time buses spend at stops and improve services still further. It will also help make drivers' jobs easier and safer."
Each week day 6,500 London buses carry about 5.4m passengers on more than 700 different routes.