(TMZ) -- Justin Theroux's war with his NYC neighbor is actually pretty one-sided -- Justin does all the damage and the downstairs neighbor's left to deal with it ... according to new legal docs filed by the guy.

Norman Resnicow responded to Theroux's recent harassment lawsuit against him, claiming he and his wife have been terrorized by the actor's rude behavior at their Greenwich Village co-op for years ... including endless barking from his rescue dogs left isolated for long periods, dropping heavy weights on the floor at 3 AM and angry outbursts.

Despite all this, according to the docs, Resnicow has actually had Theroux's back in the past -- including when he was the co-op president -- and the two have a history of "friendly relations and cooperation." That's why he calls Justin's allegations against him ... "as fictional as the television series in which he recently starred."

For example, Norman claims Jennifer Aniston's hubby asked him to help quietly kick Justin's ex-gf out of his apartment.

Resnicow says he's been nothing but helpful to Justin, yet J.T. won't return the favor by soundproofing his unit, and that's why Resnicow has pitched a fit from time to time.

TMZ reached out to Justin, so far no word back.

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