The actor, who starred alongside the Oscar-winning actress in their 2011 film "No Strings Attached," was responding to Portman's recent interview in Marie Claire UK.
In the interview, Portman revealed that Kutcher's salary on the film was more than three times what she was paid.
"I wasn’t as pissed as I should have been," said the "Jackie" star, 35. "I mean, we get paid a lot, so it’s hard to complain, but the disparity is crazy."
After Portman's comments were published online, Kutcher took to Twitter to show his support for his former co-star.
"So proud of Natalie and all women who stand up for closing the gender pay gap!" he wrote, sharing a link to the article.
So proud of Natalie and all women who stand up for closing the gender pay gap! https://t.co/AV1uYY6KIe— ashton kutcher (@aplusk) January 11, 2017
In the article, Portman said she didn't think the wage disparity arose because for similar roles "women and men are more or less capable."
"We just have a clear issue with women not having opportunities," she said. "We need to be part of the solution, not perpetuating the problem."