Some early morning, late evening, weekend and lesser used bus services are being dropped in Swansea due to council cuts in subsidies.
Operator First Cymru said there would be new timetables on around 25 routes in the city from Sunday.
Swansea council said "ongoing budget pressures" meant it had reduced transport subsidies by £200,000 to about £1.4m this year.
It said priority had been given to retaining core daytime services.
First Cymru said it would attempt to run some services that were subsidised on a commercial basis but they would only be viable if supported by the public.
Some services will see the number of buses each day reduced, and others will see altered routes, while times have been changed for other journeys.
Changes affect routes across the city, Gower and to neighbouring counties.
A council spokesman said the decision to reduce subsidies was taken at a full council meeting in February.
"We are continuing to work with public transport providers to ensure Swansea has the best possible public transport network and this year alone Swansea council is investing more than £1.4m subsidising local transport services.
"To minimise the impact on passengers priority has been given to retaining core daytime services and as a result, only some Sunday, evening, bank holiday, early morning and off peak shopping journeys have been affected.
"Ultimately bus service providers decide whether or not to continue a service if a subsidy is withdrawn."
First Cymru is advising passengers to check its new timetables.
Commercial director Kevin Hart said: "Local people will be aware that the council, while continuing to support a number of bus routes, has had to make some savings in light of the current economic conditions.
"The number of services they are supporting has fallen, but by working together we have tried to minimise the impact of these changes on customers.
"Where we have been able to we have taken a risk, turning previously supported routes and journeys into fully commercial ones.
"We hope that customers will use these services to ensure that they remain commercially viable in the long term."