Union walked the carpet at the Metropolitan Museum of Art wearing a custom white ensemble that she revealed to ET's Rachel Smith took "1,400 hours" to create.
"We've been working on this since April, and it's all about change," she shared. "Every step it changes...all the colors change, just like America."
Union also shared that, for the Gala, she "wanted to honor some folks that were very inspiring to me."
The actress walked the carpet solo tonight, and explained to ET why husband Dwyane Wade wasn't making a fashion statement of his own: "He decided to watch the Bears game instead. Too bad they lost!"
Earlier on Monday, Union showed off her special look in "tribute to an American Icon."
"I wanted to honor Ms. Cicely Tyson and the dress she wore to the 1974 Emmys (one of my favorite red carpet looks of all time) by asking one of my favorite designers to create this dress as an homage to Ms Tyson," she wrote on Instagram. "@samanthablacknyc took the challenge and ran with it. Here is the piece she created. THANK YOU!!! Give @samanthablacknyc a follow & some love. #MetMonday#BlackDesigner🖤."
This year's Met Gala theme is "In America: A Lexicon of Fashion," an exploration of the nation’s sartorial identity and a deep dive into American ingenuity. According to the press release, the exhibition opens Sept. 18 and will highlight "everything from the luxe ease of Halston's '70s glamour to Rodarte's ethereal edge and Kerby Jean-Raymond's powerful political vision for Pyer Moss."