May 20, 11:30 PT -- Justin's neighbor, Norman Resnicow, tells TMZ ... "I have acted for one purpose only, which remains to assure my and my wife's health and safety. Smart people can work [this] out and both my wife and I and Justin are represented by smart people."

Justin Theroux is in a nasty war with a neighbor who's threatened to cut the actor's water and electricity ... according to a new lawsuit.

Justin and Jennifer Aniston are making renovations to their Greenwich Village co-op, and he says that's not sitting well with the guy who lives downstairs from them. In the suit, Justin says the neighbor's been bitching that the sound-proofing materials going in Theroux's pad aren't good enough. He's demanding Justin spend an extra $25-30k to upgrade.

Justin says when he refused to give in, the neighbor launched a nasty campaign ... including threats to cut the water and electricity lines that run to Justin's roof deck, harassing construction workers at the home and perhaps worst of all ... threatening to prune Justin's ivy.

Them's fighting words in the Village!

Theroux says the guy even threatened to set off a PR nightmare when he emailed ... any legal action would be "opening the door to a broad range of allegations that would become publicly available, and which would not please Justin or his wife or their respective publicists."