The actor and his model girlfriend hit the New York City streets Saturday ... with the two hopping on bicycles and flying down the streets -- Leo in his usual incognito mask and hat combo.

Check out the pics ... while he's trying not to attract too much attention, he needs his nose to breathe during the physical activity -- and, it's pretty clearly him even with the mask on.

Vittoria's also keeping a low profile ... thick black sunglasses and a hoodie pulled up, covering her hair. In one pic, Leo's niece Normandie Farrar can be seen riding with her unc and his GF, so they weren't totally alone.

Of course, LD and VC are back stateside after enjoying a rip-roaring European vacation. The two were just in Italy earlier this month ... snuggling up together in Rome and later going out to dinner with Vittoria's parents.

Leo got a red wine for the table which cost more than $27k ... though when your cleintele ranges from Jeff Bezos and Lauren Sanchez to Katy Perry and Orlando Bloom, ya gotta stock up on the good stuff.

The two have spent extensive time together and with each other's families, with Normandie being the most recent member of DiCaprio's family to meet Vittoria ... so, this is defintely the real deal.