The four additional services will include an 08:54 from Aberdare, meaning customers can travel by train into Cardiff before 10am for the first time.
It is hoped that demand will prove sufficient to keep the extra services on a permanent basis. Arriva Trains Wales have been working with local AM Vikki Howells, who conducted a survey showing 95 per cent of people in the area said they would like a more regular Sunday service.
They will be in place for the 3rd, 10th and 17th of December initially to assess the demand for them.
"We've listened to what our customers wanted and are delighted to be able to run these extra services in the build up to Christmas," said Arriva Trains Wales Managing Director Tom Joyner.
"We're really grateful to Vikki Howells AM for her work on this survey for her community.
"It will give more options for those travelling into Cardiff for their Christmas shopping, social events and for work.
"We'll look closely at how successful this is as we look to make a business case to make them permanent.
"It's vital we target our resources where they are most needed to best provide for the communities we serve."
The additional services mean there will be hourly services to Cardiff until 12:53 every Sunday and hourly services back from 15:41 to 19:41.
Cynon Valley AM Vikki Howells said: “Many constituents got in touch to tell me about the difficulties they faced in getting to and from Cardiff by rail on Sundays.
“I surveyed Cynon Valley residents on whether they wanted extra Sunday services – an overwhelming number of respondents said they did.
“I thank local people who took the time to express their views, as this helped me make the case with Arriva Trains Wales.
“The extra services announced for December certainly represents an early Christmas present.
“However, I hope these extra services are well-used, so that Arriva can clearly see the need and make this a long-term arrangement”.
(New Arriva Trains Wales Managing Director Tom Joyner)
The current Wales and Borders Rail Franchise drew up specific requirements for the frequency of trains on each line for any given day. Arriva Trains Wales had already exceeded the requirement, however is continally reviewing its timetable to best meet the needs of our customers.