The 21-year-old actor sported a silver, sparkly H&M dress, while the 25-year-old actress donned a long black suit jacket.
The duo, who appears as an onscreen couple Betty and Jughead on the CW drama series Riverdale, kept it close as they enter the Metropolitan Museum of Art.
Lili teased her appearance at the Met Gala via Instagram earlier on Monday, sharing a screenshot from a 2017 tweet in which a fan asked her if she was attending.
She responded at the time, “Psh, I wish. In my dreams.”
The relationship rumors between the couple first sparked in July 2017 after they were caught holding hands and kissing during a cast trip to Comic-Con in San Diego.
The pair has never spoken about their relationship in public, they have been seen packing on the PDA on multiple events, including on the beach in Hawaii in January and on the streets of Paris in April.
Most recently, Lili and Cole were seen enjoying a trip to Mexico.
Last month, during an interview with Seventeen magazine, Lili explained her decision to keep her relationship with Cole private.
Every time someone asks me about it, I make a decision then and there whether I’m comfortable. It’s not like I’m in some kind of contract, but I’m not in a place where I want to talk about it and that’s totally fine. It’s called a private life for a reason — it’s mine and it’s special and sacred. My relationships are between me and whomever I’m with, not between me and the world.