Brand told The Sun newspaper in London he developed a crush on Kunis when they co-starred in the 2008 comedy "Forgetting Sarah Marshall," but she wasn't interested because she was in a long-term relationship with former child actor Culkin.
Kunis and Culkin dated for nearly nine years and announced their breakup in 2011. She is now dating her "That '70s Show" co-star Ashton Kutcher.
Brand is divorced from pop singer Katy Perry.
"She's gorgeous. She's so gorgeous," Brand said of Kunis. "I was chatting her up, and got the conversation around to, 'Have you got a boyfriend?' and she went, 'Yeah.'"
The comedian said he was stunned to discover who Kunis was dating.
"So, you know that feeling of 'How long do I have to maintain an interest in this conversation now?' It's 12 seconds. She's talking about her boyfriend 'Mac'. Eventually, she said 'My boyfriend's coming tomorrow.' She was excited -- and when he came it was not 'Mac', it was Macaulay Culkin! He'd got longer. Like a veal calf. Like he'd been grown in a crate. All pale and scared of himself, like a shaved horse. There was a horror about him! Macaulay Culkin! The little lad out of 'Home Alone!' She's [bleeping] him!"