Zac Efron Touches Down in Rio ... First Stop, Simone Biles

Zac Efron stuck his landing in Rio, and immediately hooked up with Simone Biles ... moments after she nailed down her 4th gold medal.

The timing is no accident -- Simone crushed hard on Zac and the two exchanged some flirty tweets after The Final Five won gold in the team competition. Next thing ya know, BOOM ... Zac's flying south, and getting a smooch from Simone on the 'Today' set.

It's a good guess NBC paid for Zac's first class ticket to Rio.

Sure, it's just a kiss on the cheek, but at this point ... would you bet against Simone ending up with the gold?