In case you missed it, he filled in for Simon, who was unable to make it to the Kansas City auditions because he fell ill with bronchitis.
Simon said he called on his best pal Louis because he trusts no one more than him (aside from L.A. Reid of course!) when it comes to music. But who exactly is Mr. Louis Walsh?
We did a little digging and here are 5 Fun Facts About Louis:
1. He Was a Precocious Teen: Louis Walsh, born Michael Louis Vincent Walsh, started his career at the young age of 15.
2. He Knows Talent: Louis has managed multi-million selling artists such as Boyzone, Westlife (through whom he met Simon Cowell), Girls Aloud, Samantha Mumba, Shayne Ward (a former THE X FACTOR UK winner), and Johnny Logan.
3. He Knows TV: He's also served as a judge on Britain's Got Talent, You're a Star, and Popstars: The Rivals.
4. He's Famously Outspoken: He once called the Spice Girls' "past their sell-by date" and fellow UK judge Tulisa Contostavlos a "chav in a tracksuit." (chav loosely translates to "white trash" here).
5. He's Well-Respected. Biography channel calls him "the undisputed king of the Irish pop scene" and he won the 2009 Mayo Man of the Year award.
So, how do you think Louis Walsh did on tonight's episode? Hit the comments and let us know your thoughts.