The 39-year-old actor spoke out Sunday after photos surfaced of him boarding a private plane with a "mystery" woman June 26 in Los Angeles.
"You should have heard how upset Mila was that I spent the day with our cousin. Sorry aunt Jodie these magazines lack integrity," he wrote to his 18.2 million Twitter followers.
Star magazine published pictures of Kutcher with the woman in its July 17 issue. The publication also revisited rumors Kutcher cheated on ex-wife Demi Moore on their sixth wedding anniversary in September 2011.
Kutcher and Moore split in November 2011, and finalized their divorce in 2013. The actor addressed his reported infidelity to Moore while accepting the 2017 Robert D. Ray Pillar of Character Award in April.
"[I'm] probably the first guy to get this award for character who had his name splashed across every gossip magazine as an adulterer, like, five years ago," he said in his speech.
"Character comes when those magazines tear you apart for something you may or may not have done and you gotta go out and perform tomorrow with everyone looking at you like you might be an adulterer," the star added.
You should have heard how upset Mila was that I spent the day with our cousin. Sorry aunt Jodie these magazines lack integrity. pic.twitter.com/tvKdGoqRnx— ashton kutcher (@aplusk) July 9, 2017
Kutcher and Kunis married in July 2015, and share 2-year-old daughter Wyatt and 7-month-old son Dimitri. The actor said in an interview with The Howard Stern Show in June that his romance with Kunis was "weird and fast."