John Starks & Anthony Mason -- Our Pizza Joint ... IS OPEN FOR BUSINESS!!

There's a new basketball team doing business in Harlem ... a couple of former New York Knicks -- who just officially OPENED their first pizza joint together ... TMZ Sports has learned.

John Starks and Anthony Mason finally cut the ribbon on the first NYC location of Custom Fuel Pizza -- a chain that started in D.C.

The duo is pumped about being investors in the franchise (they previously told us they want to challenge Peyton Manning for the pizza title) ... and last night at the grand opening party, John and Anthony gushed about how they plan to change the pizza game forever.