The introduction comes after close work between ATW and local AM Vikki Howells on the matter and a successful pilot during December 2017.
It was described as a "really important improvement for customers" by Arriva Trains Wales Managing Director Tom Joyner, who said it would help those wanting to travel for work and leisure.
Vikki Howells AM had been calling for the extra services and thanked the public for their contribution and Arriva Trains Wales for working with her.
"I'm delighted to be announcing these additional services from Aberdare, which will mean hourly services on a Sunday morning with trains arriving before 10am, on a permanent basis, for the first time," said Ms Howells.
"This demonstrates what you can achieve with people power,"
From May onwards, Aberdare will two extra services into Cardiff every Sunday morning and two back every Sunday evening.
It means people in Aberdare, Cwmbach, Fernhill, Mountain Ash, Penrhiwceiber and Abercynon will have hourly trains into Pontypridd and on to Cardiff from 08.57 through to 12:54 for the first time.
And coming home they will have hourly options with trains at 15:41, 16:41, 17:41, 18:41 and 19:41.
Mr Joyner said: "We're really pleased to be introducing these extra services for our customers in the Cynon Valley.
"I'd like to thank Vikki for the hard work she has done on behalf of her constituents.
"We've really listened to our customer feedback which shows a real need for these services, backed up by the results of a pilot study."