First South Yorkshire said from August 1, the 34 new buses would be incorporated into its 106-strong fleet in Doncaster to offer a "new, high standard" across the network.
All the new vehicles, which include 22 double deck buses and 12 single deckers, will be low-floor to provide easy access and will be fitted with CCTV cameras.
Company managing director Bob Hamilton said each bus was equipped with an engine which met strict new EU guidance on pollution and carbon emissions.
"First is carrying out a renaissance or 'fresh start' of how it operates its buses in Doncaster.
"A big part of this will be incorporating 34 modern low-floor buses into our Doncaster fleet.
"We will also be ensuring all our drivers are trained to at least NVQ Level 2 in customer care and are offering to refund passengers if our buses are 20 minutes behind the timetabled schedule owing to factors we have some control over.
"This is all good news for First customers in Doncaster and follows on from our recent partnership work with the Mayor to test new routes and promotions in Doncaster."
The new buses are set to be unveiled on Doncaster Racecourse today by Doncaster Mayor Peter Davies and other representatives from the council and transport executive.
Mr Davies said: "I am delighted with the news that First is to unveil a new fleet of buses.
"This shows their commitment to transport in Doncaster, and is proof of the success of the talks held with Sir Moir Lockhead, chief executive of FirstGroup, earlier this year."
First employs 237 drivers in Doncaster alongside 72 other members of staff who provide engineering, managerial, cleaning and administrative support.
The company's depot is in Leger Way, opposite the racecourse, and its buses are responsible for an average 320,000 passenger journeys a week.