The 46-year-old Grant, who wore her natural, silver hair tied back in a bun with her bangs down, passingly resembled Mirren, and the internet went wild with the misunderstanding.
Mirren walked her own red carpet in New York City on Wednesday, at the premiere of her new movie, The Good Liar, and she stopped to talk with ET -- alongside her co-star, Ian McKellen -- where she addressed the mix-up.
"I saw that," Mirren said, referring to the Twitter furor that erupted over Reeves' rumored flame. "That was very flattering on me, you know, because she's obviously lovely."
While the pair aren't dating -- in fact, Mirren has been married to director Taylor Hackford for 22 years -- the Oscar winner does know Reeves, and has nothing but love for the the John Wick star.
"I do know Keanu very well. He did a film with my husband and he is just the most adorable, lovely person," Mirren said, referring to the 1997 horror thriller The Devil's Advocate. "So she's a lucky girl and I'm sure that he's a lucky boy."
Reeves and Grant have an extensive history together, with the L.A.-based artist having worked with Reeves on his 2011 adult picture book, Ode to Happiness, written by Reeves and featuring illustrations by Grant.
In 2016, the pair collaborated again on another book, Shadows, and posed together on the red carpet at the UNAIDS Gala during Art Basel 2016 in Basel, Switzerland. The two also started a publishing company together the same year.
Meanwhile, The Good Liar marks the first time Mirren and McKellen have ever worked together on screen during their storied, celebrated careers. According to McKellen, a lot of the credit goes to the film's Oscar-winning director, Bill Condon, whom the star has worked with frequently.
"When Bill Condon asks you to be in the movie you'll do it, even if Helen Mirren isn't in it. But then it turned out she was," McKellen explained. "It was a double whammy for me."
The Good Liar -- in which McKellen stars as a career con artist who targets Mirren, a recently widowed woman now worth millions, by posing as a suitor through an online dating site -- hits theaters Nov. 15.