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Liam Hemsworth is the one who's pulling the plug on his marriage, because TMZ has learned he just filed for divorce from Miley Cyrus.

Liam filed legal docs in L.A. ... citing the standard "irreconcilable differences."

Sources connected to the couple tell TMZ, Liam and Miley had a prenup, keeping their earnings during the marriage separate, so there won't be a fight over money. And, since there are no kids, we're told the divorce will be easy breezy.

The handwriting was on the wall for weeks ... Miley and Liam announced their separation August 10, after less than a year of marriage. Our sources tell us Liam is "over the marriage" and just wants to move on.

The day before telling the world she split from Liam, Miley was spotted making out with her longtime friend, Kaitlynn Carter, at Lake Como in Italy. There were allegations of infidelity, although it appeared the couple had split months before.

The date of separation is relevant, not so much legally, but in terms of PR. In the docs, filed by disso queen Laura Wasser, she does not list a date of separation ... it reads TBD -- to be determined.

Here's what's interesting ... our sources say, despite appearances, Miley had made overtures as recently as this week, saying she wanted to work on the marriage, but Liam was steadfast that it was over. We're told fact is, Liam's people said he would be "insane" to jump back in.

Liam and Miley had been drifting apart for some time ... Liam was photographed in June grocery shopping without his wedding ring.

The breakup started out amicable, but it quickly turned nasty, with claims of drug use and infidelity. She released a song last week, "Slide Away," which we're told is her version of the split, blaming it on Liam's alleged penchant for whisky and pills ... something he strongly denies.

Miley and Liam tied the knot back in December, hosting friends and family for a wedding celebration at their home in Nashville.