A 21-year-old girl is quoted as saying that she "made love on his living room couch while Demi was out of town."
In a statement, Kutcher's lawyer, Marty Singer, tells UsMagazine.com, "Star Magazine continuously publishes lies about Ashton Kutcher and many other celebrities. This is not the first, nor will it be the last time they engage in reckless conduct."
It is the second story Star has run alleging that Kutcher, 32, had an affair. Another article this month claimed he had a fling with "a sexy, young, 20-something."
Kutcher quickly took to Twitter to challenge the report (which Star has refused to retract): "I think Star magazine calling me a 'cheater' qualifies as defamation of character. I hope my lawyer agrees," he wrote. "STAR magazine - you don't get to stand behind 'freedom of the press' when you are writing fiction."
Moore, 47, Twittered him back, "Excellent point my love!"
Kutcher and Moore, who first met in 2003, celebrate their five-year wedding anniversary Sept. 24.
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