The STV film was honoured alongside acclaimed BBC drama I May Destroy You and Russell T Davies, creator of recent Channel 4 hit It’s A Sin.
Produced and filmed entirely in Scotland, Glenda Jackson has already won an Emmy and a BAFTA for her performance in Elizabeth is Missing as an elderly woman living with dementia who is haunted by two mysteries in her life.
The film, which was adapted by Scottish screenwriter Andrea Gibb, whose previous credits include Dear Frankie and Nina’s Heavenly Delights, from the best-selling novel by Emma Healey.
It was filmed on location in Paisley, with the Renfrewshire town standing in for a 1940s English setting in flashback scenes.
Elizabeth is Missing was the first screen role for more than 25 years for Jackson, who left the profession in 1992 to begin a career as an MP, and returned to stage work in 2015.
Initially shown on BBC One, the film premiered in the United States earlier this year as part of PBS’s Masterpieces series.
The film was named best single drama in the awards, which saw the Wales-set coming-of-drama In My Skin named best drama series and I May Destroy You named best mini-series, while its creator and star Michaela Coel was named best writer and best actress, ahead of Jackson and Daisy Edgar-Jones, star of Normal People.The awards also recognised The Masked Singer, Loose Women, The Young Offenders and Casualty.
The RTS judges hailed Elizabeth Is Missing as “a beautiful film full of layers, all built around an intensely emotional and compassionate central performance.”
In the film, Jackson plays Maud, a woman whose search for her missing friend Elizabeth takes on a poignant urgency as dementia threatens to erase all the clues.
Sarah Brown, creative director of drama at STV Studios, said: “We’ve been blown away by the response to Elizabeth is Missing and to be recognised by the Royal Television Society for single drama in such esteemed company is incredibly gratifying.
“We knew we were working on something special when we set out to adapt Emma Healey’s wonderful novel, and I’m so proud of our hugely talented writer, director, cast and crew for bringing this story to life so beautifully.
"Special mention must go to Glenda Jackson for her breath taking performance as Maud and the sensitive and powerful way she portrayed a woman living with dementia, which I know has resonated with so many viewers.”
Gibb told her Twitter followers: “Working with the great Glenda Jackson on Elizabeth Is Missing was the kind of once of a lifetime experience we'll never forget. Huge thanks to her.”
Speaking about her role last year, Jackson said: “Elizabeth is Missing was a wonderful film to work on and the book by Emma Healey has been a revelation to me.
“There are so many families affected by dementia and the issue is wide spread, as far as the eye can see. I hope the film itself and this recognition will help to highlight the daily struggles that these families face.”