The 57-year-old Australian was awarded for her risque role in the erotic Babygirl, where she plays a hard-nosed CEO who jeopardises both her career and her family by having a toxic affair with a young, manipulative intern played by Harris Dickinson.
Speaking to the audience at the festival on Saturday night via a note read out by Babygirl director Halina Reijn, Kidman said her "heart is broken".
It read: "Today I arrived in Venice to find out shortly after, that my beautiful, brave mother Janelle Ann Kidman has just passed.
"I am in shock and I have to go to my family, but this award is for her, she shaped me, she guided me, and she made me.
"I am beyond grateful that I get to say her name to all of you through Halina, the collision of life and art is heart-breaking, and my heart is broken.
"We love you all."
The 81st edition of the film festival closed on Saturday, with Pedro Almodovar's English-language debut The Room Next Door winning the festival's most prestigious prize, the Golden Lion, which is awarded for best film.
Director Halina Reijn accepted an award for best actress on behalf of Nicole Kidman. (AP PHOTO)
Starring Tilda Swinton and Julianne Moore, the film received an 18-minute standing ovation when it premiered at Venice earlier in the week - one of the longest in recent memory.
Almodovar is a darling of the festival circuit and was awarded a lifetime achievement award at Venice in 2019 for his bold, irreverent and often funny Spanish-language features.
Now aged 74, he has decided to try his hand at English, telling reporters that it was like science fiction for him.
While The Room Next Door had been widely tipped to win, the runner-up Silver Lion award was a surprise - going to Italian director Maura Delpero for her slow-paced drama set in the Italian Alps during World War II - Vermiglio.
The best director award went to US filmmaker Brady Corbet for his three and a half-hour-long movie The Brutalist, the epic tale of a Hungarian Holocaust survivor played by Adrien Brody, who seeks to rebuild his life in the United States.
The festival marks the start of the awards season and regularly throws up big favourites for the Oscars, with eight of the past 12 best director awards at the Oscars going to films that debuted at Venice.
The prize for best screenplay went to Murilo Hauser and Heitor Lorega for I'm Still Here, a film about Brazil's military dictatorship, while the special jury award went to the abortion drama April by Georgian director Dea Kulumbegashvili.
Frenchman Vincent Lindon was named best actor for his performance in the movie The Quiet Son.
The Venice jury this year was headed by French actress Isabelle Huppert.
with PA and Reuters
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