NY Giants Linebacker -- TREAT. YO. SELF. ... $55k Jewelry Run

New York Giants linebacker J.T. Thomas believes in the "Treat Yo Self" mentality ... because after he inked his new $12 MILLION contract ... he picked up $55k in brand new bling ... TMZ Sports has learned.

Thomas hit up Avianne and Co. in NYC -- and scored himself an 18 karat gold Cuban link chain for $35k.

And because he didn't want that chain to get lonely, he got a 2nd piece ... a 25 karat ruby, surrounded by diamonds ... which ran him another $20k.

Tom Haverford would be proud ...