The 'Lolita' star's son announced the sad news that his parent passed away due to pneumonia complications not relating to COVID-19, after a lengthy health battle.
The 'Lost Boys' actor hailed his mom ''an extraordinary woman who led an extraordinary life''.
He wrote on Twitter and Instagram: ''Early this morning my mother, Shirley Douglas, passed away due to complications surrounding pneumonia (not related to COVID19).
''My mother was an extraordinary woman who led an extraordinary life.
''Sadly she had been battling for her health for quite some time and we, as a family, knew this day was coming.''
Although Shirley's death was not related to the coronavirus, Kiefer had a message of support to those who have lost relatives to the respiratory disease.
He added: ''To any families who have lost someone unexpectedly to the Coronavirus, my heart breaks for you, please stay safe.''
As well as featuring in Stanley Kubrick's adaptation of the novel 'Lolita' in 1962, and 'Dead Ringers' in 1988, Shirley was known for her activism against the War on Vietnam, and the battle for public healthcare.
In 2012, she was honoured with a Queen Elizabeth II Diamond Jubilee Medal.
Whilst the Canadian-born star received an Officer of the Order of Canada in 2003.
Shirley is survived by Kiefer, 53, and his twin sister Rachel from her marriage to actor Donald Sutherland - who she was married to between 1966 and 1970 - and Thomas Emil Sicks from her first marriage to Timothy Emil Sicks.
Kiefer's passion for acting came from his mother
He recently recalled: ''I was raised by my Mum, and my Mum is a very successful theatre actor in Canada.
''I have a twin sister and she and I would finish school, and we would go to the theatre, do our homework, wait for my mum's performance to finish, and then we'd go home. That was our day.
''I spent so much time as a child in the theatre. I could tell that people who worked in the theatre were different.
''When I would go visit other friends, at their house, their parents and their friends weren't the same. They weren't just colourful, they weren't just funny.
''I knew I wanted to kind of live in that world, I didn't know what I wanted to do.''
And Kiefer's mind was blown when he saw his mom on stage for the first time.
He added: ''My mother did a production of 'Virginia Woolf', and it was the first time I had seen either one of my parents where I didn't recognise them anymore and it blew my mind. It was like magic, and that's when I first got excited about what I thought was interesting about acting.''
Here's what the public had to say about the death of Phil Spector.
Lana Del Rey takes her 60s vintage aesthetic to the extreme with the video for new single 'Chemtrails Over The Country Club'.
As negotiations continue, it's clear that the UK government doesn't have everyone's best interests at heart.
The singer awkwardly responded to potential backlash regarding her new album cover.
Five years ago, on the day of his 69th birthday and two days before his death, David Bowie released his 25th, and final studio album, 'Blackstar'.
Nobody is impressed by Demi "coming to the rescue".
Let's leave gatekeeping in 2020.
These are the albums we're looking forward to most this month.
When medical student Courtney Homes approaches fellow student Jamie with the an intriguing prospect of...
When Thunder is abandoned into the street by his owner as a kitten during a...
Like an ancient Roman version of 2012, this disaster epic is a pure guilty pleasure,...
After being enslaved, Milo is made into a gladiator with indomitable strength. He is forced...
A terrific story is compromised by the demands of commercial filmmaking, adding action-thriller scenes to...
In a grand castle located in the beautiful countryside, Justine and Michael have married. They...
Marmaduke is perhaps the world's most famous Great Dane, he was originally brought to life...
Watch the trailer for Monsters Vs Aliens. When a UFO hits down to Earth in...