Mirren, 69, and her American director husband Taylor Hackford, 69, have recorded a video message of support for Senator Dario Stefano, who will fight for the nomination of an alliance of centre-left parties in a primary scheduled for November 30th.
Expressing their support and admiration for Stefano, the couple, who have a holiday home in Puglia, said he had done "extraordinary work for agriculture and the environment," Italian news agency Ansa reported.
Stefano, 51, is a member of the Left Ecology Freedom party but is considered a centrist in his politics. He sits on the regional board of the Italian employers' organisation Confindustria.
Mirren, who flirted with the extreme left fringe of British politics in her youth, told an interviewer last year that she had always been a "pretend" leftie.
"I was a pretend left-winger because it was more interesting than being a right-winger," she told Time Out magazine.