The former One Direction singer, 24, sported a sparkly, bedazzled Los Angeles Dodgers uniform identical to the iconic ensemble Elton wore for his shows at Dodger Stadium in 1975.
Harry's custom-made ensemble was created by Gucci, and didn't go unnoticed by the "Rocket Man" hitmaker himself.
"Now that's what I call a Halloween costume," the music icon, 71, wrote alongside a series of snaps featuring Harry posing with his tongue stuck out.
The "Sign of the Times" hitmaker was dressed appropriately for the Halloween event, as it was held on the same night as the World Series baseball game between Los Angeles Dodgers and the Boston Red Sox, which ended up being the longest game in World Series history after stretching into the 18th inning.
Other high-profile names at the Casamigos bash included Cindy Crawford, who was dressed as Debbie Harry, husband and Casamigos co-founder Rande Gerber, who was David Bowie, and their supermodel daughter Kaia Gerber, dressed as Joan Jett.
Paris Hilton was a Furby, while Nicky Hilton dressed as her sister Paris, Sean "P. Diddy" Combs was an airline pilot, Ryan Seacrest dressed as designer Karl Lagerfeld, and Olivia Munn went as Peik Lin from the movie "Crazy Rich Asians".
"Happy Halloween to everyone, but especially to all you crazy rich Asians!!" Olivia captioned a snap of her in costume on Instagram.
Model Cindy and husband Rande were joined by George Clooney as they flew to the Casamigos Tequila halloween bash in Las Vegas on a private jet, the day after the L.A. party.
Wearing matching pilot uniforms, George and Rande posed in the plane's cockpit, while Cindy wore a 60s style air hostess uniform, complete with knee-high PVC white boots.