Everyone who lives or works there can now benefit from a free test.
It follows a similar pattern in Liverpool, and members of the armed forces are again involved alongside the local authorities.
The aim is to identify the most positive cases and to stop the spread of the disease without control.
The Merthyr Tydfil Rhydycar Leisure Center opened on Saturday and more sites will go live later this month.
Lateral flow tests are used – they give a result in about 20-30 minutes and do not need to be sent to a laboratory.
People who test positive will also receive a traditional swab test – which has a higher degree of accuracy – and will be asked to return home to self-isolate.
Health Secretary Matt Hancock offered the rapid tests to all local authorities in England, as well as to decentralized nations.
Merthyr Tydfil had the worst infection rate in the UK at the start of November, but the recent ‘firewall’ lockdown in Wales appears to have had a significant effect.
Cases in the region have fallen from around 770 per 100,000 population to less than 260.
Blaenau Gwent is now Wales’ virus hotspot, with more than 350 cases per 100,000.
The overall number of cases in the country fell in the 10 days leading up to Friday, and there are also signs of a slowdown in hospital admissions.
The country’s seven-day incidence rate is now around 160 cases per 100,000 population, according to Prime Minister Mark Drakeford.
Despite the success, he said there was “no room” for further relaxation of the rules in Wales.
“We need everyone, each of us, to play our part to ensure we continue to take advantage of the progress of the firewall and bring down coronavirus cases,” Mr Drakeford said on Friday.
He added, “If we do all of this together then we can have a way until Christmas and we hope to see family and friends during this festive time. ”
Mr Drakeford said talks on a UK approach to Christmas rules are scheduled for next week.