The former child star has shacked up with the Babyshambles frontman at his Paris home, The Sun reports.
"Macaulay and Pete may seem an odd couple but they’ve got loads of interests and life experiences in common," a source told the paper.
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Culkin, now age 32, has remained relatively out of the spotlight since quietly splitting from Mila Kunis in January 2011. In August 2012, he was photographed attending longtime pal Natalie Portman's wedding, in Big Sur, California.
Nine months ago the National Enquirer alleged that Culkin was a heroin addict, who spends $6,000 a month on prescription painkillers.
However, reps for the actor released a statement to The Huffington Post denying the Enquirer's entire report:
The report in the National Enquirer that Macaulay Culkin is addicted to Heroin and assorted hallucinogenics is not only categorically without merit, but it is also impossibly and ridiculously fictitious. We beseech the responsible media to consider the source and its reputation and to please not perpetuate this destructive and insulting story by pursuing it any further.