ITV's new drama, The Pembrokeshire Murders, launched on Monday night with 6.3 million viewers.
Over a third of people watching TV across all channels tuned in to see the first episode of the new three-part true crime series, depicting the pursuit of a cold-blooded serial killer.
This is the channel's biggest overnight audience for a first episode of a new drama since 'The Durrells' in 2016.
Following the first episode, audiences praised the drama on social media for its "sensitive" portrayal of the story.
TV personality Carol Vordeman, who grew up in Prestatyn, said: "Such a sad story, but portrayed sensitively and beautifully."
"Superbly acted throughout, beautifully shot and made me properly proud to be Welsh", said Scott Arthur.
"Excellent first episode", said Owen Thomas.
Cory Shipp said that it was a "drama done right" and gave her "such love for Wales".
Alun Saunders added that it was "refreshing to be able to watch quality Drama set in Wales" and "crammed with brilliant Welsh talent".
Pembrokeshire's MP, Stephen Crabb, who said he remembers the reaction of the local community when the murders took place has also praised the first episode.
He said: "Impressive dramatisation on ITV this evening of the remarkable Dyfed Powys police investigation into the Pembrokeshire murders.
"Those events made a big impression on all of us growing up in Pembs in the 1980s. Hard police work and science finally secured justice 25 years later."
The series, starring Luke Evans and Keith Allen, continues on ITV on Tuesday evening at 9pm and concludes at the same time on Wednesday.