STAGECOACH is closing its depot in Brynmawr in the Gwent Valleys, threatening 77 jobs.
A drop in funding from the Welsh government and lower local authority investment in bus services are being blamed by the company for the changes.
Routes in Monmouthshire, Torfaen, Caerphilly and Rhondda Cynon Taf will be affected.
Around 900 people are employed at seven Stagecoach depots in Wales according to the company, which also says it operates almost 400 vehicles and carries about 27 million passengers annually.
"Sadly it is impossible to continue delivering the same level of services for passengers when we are seeing cut after cut in investment in local bus services," South Wales managing director John Gould is reported as saying.
"The result is that some of our routes are not sustainable and we have to act in order to ensure we can continue to deliver services for customers across the rest of our network over the long-term."
In response, the Welsh government said: "The new reimbursement rate for bus operators was set following an independent review to ensure operators such as Stagecoach were 'no better and no worse off' by taking part.
"As yet, Stagecoach has not formally informed Welsh government of the threatened closure or service cuts."
Blaenau Gwent Council has said it has not cut its subsidy to Stagecoach.