The hashtag #PhelpsFace emerged after Phelps, 31, was caught on camera glowering with his hood over his head and his hands in his pockets as he prepared to race in the 200-meter butterfly semifinal.
Nearby Phelps was le Clos, who prepared for the same race by shadow boxing and smiling in front of Phelps.
Phelps finished second in the race while le Clos finished fourth.
“I was trying to not really even look at him," Phelps told NBC's Michele Tafoya after the race. “He does his thing, I do my thing."
The two will compete again tonight in the 200-meter butterfly final. In the meantime, people on the internet can’t get enough of Phelps’ expression.