Appearing as Tony Stark, his "Iron Man" alter ego, Downey presented a real bionic arm to Alex, a 7-year-old boy who loves superheroes and was born with a partially developed right arm.
Albert Manero, a college student who builds and donates low-cost, 3-D printed bionic limbs to kids around the world, put the two together as part of Microsoft's Collective Project.
And their meeting was captured on video, which Downey posted on Twitter today.
"Honored to present a bionic #IronMan arm to Alex through @MSOneNote’s #CollectiveProject," Downey tweeted. "Check the bowtie. #dapper."
In the video, you can see Downey greeting young Alex in a dapper red bow tie.
As the pair sit down to check out the bionic arms Downey brought along in metal cases marked Stark Industries, Alex is asked if he knows who Downey is.
"Iron Man," he replies without hesitation, causing everyone, including Downey, to break into laughter.
Asked if he knows Downey's name, Alex replies, "Robert," to which Downey responds, "Great!"
The pair then put on and compare their bionic arms.
"I'm having a technical glitch," Downey tells Alex in the video, as the light on his Iron Man arm continually flashes. "Half the time I design one of these, it winds up breaking on me. But what I do is I keep working on it, kind of like you're working on it with Albert."
"He keeps working and working until he gets it right," Alex replies.
Later, Downey praises Albert.
"Albert has made it so affordable," the actor said. "I'm probably going to start farming out a lot of my tech work to Albert, too. I feel like he could cut down the price point on one of my suits, which is I guess about now a billion and a half dollars."
Check out the cute video below: