Sisqo's Threatening Legal Action Against Impostor

Sisqó's PISSED at the impostor who got all sorts of perks at New York Fashion Week when people thought he was legit ... and now the real singer's threatening to take legal action against the wannabe.

The former Dru Hill frontman tells TMZ ... he tried laughing off the episode after learning about it but Sisqó's tone changed when the guy tried cutting deals as Sisqó.

You'll recall the fake Sisqó -- reportedly Gavin Barnes -- was decked out in Sisqó's trademark outfit ... a Versace suit, blond hair and stunner shades scored front row seats to huge Fashion Week events, including Kanye West's show.

But check out what has Sisqó fuming. It just got real.