Dallas Cowboys cornerback Trevon Diggs' son, Aaiden had a priceless reaction to meeting star quarterback Dak Prescott ... by mistaking him for Patrick Mahomes!!

The whole interaction was absolutely adorable ... and thankfully, it was captured on the finale of HBO's "Hard Knocks" Tuesday night.

4-year-old Aaiden -- who refers to Dak as his "favorite quarterback in my whole entire world" -- was wearing a #4 jersey at an ice cream shop with his pops when none other than Prescott walks in the door.

And, Aaiden's reaction is incredible.

"So, you’re Patrick Mahomes??" he asked.

Dak played it off like a champ -- saying, "You think I’m Patrick Mahomes still?? Whose jersey is this??" he said as the whole room cracked up.

Aaiden then corrected himself ... and then said, "Good to see you."

Despite the confusion, Dak still hooked Aaiden up with some awesome keepsakes ... including 2(!!) jerseys -- one to wear and one to frame.

The best part -- Dak played along with Aaiden ... signing one of the jerseys as, "Not Patrick Mahomes" to prevent further mistaken identity!!

Of course, this isn't the first time Aaiden's made the mix-up ... in a previous episode, he thought his #4 jersey was actually a nod to Mahomes (also adorable).