Tim Tebow -- Hardcore MMA Training with Fighting LEGENDS

Tim Tebow has begun training with some of the baddest men on the planet ... men who revolutionized the art of hand-to-hand combat ... the legendary Gracie family.

In case you're unaware, the Gracie family basically built the UFC -- and badasses like Royce Gracie have destroyed countless opponents by using a special form of Brazilian jiu-jitsu created by his father, Helio Gracie.

Now, the New York Jets quarterback has joined up with the Gracie family at their headquarters in Beverly Hills -- where he was training with Royce's nephew Ryron Gracie and UFC heavyweight Brendan Schaub (in the grey shirt).

After the training session, the Gracie brother's tweeted about their newest pupil, "Everyone please give a warm welcome to the newest member of the Gracie Family."