Lili Reinhart Wore Vintage Emanuel Ungaro at "The Love Hypothesis" London Premiere

Lili Reinhart in vintage Emanuel Ungaro Spring 2001 at the London premiere

Lili Reinhart brought some serious vintage glamour to London this week. The actress, known for playing Betty Cooper on "Riverdale" and for films like "Chemical Hearts" and "Look Both Ways," stepped out for the UK premiere of "The Love Hypothesis," based on the bestselling Ali Hazelwood novel, alongside co-star and on-screen love interest Tom Bateman.

Reinhart wore an archival gown from Emanuel Ungaro's Spring/Summer 2001 collection, and the dress leans fully into soft, watery romance. Done in a dusty lilac-purple tone, the spaghetti-strap slip gown is scattered with hand-painted, petal-like patches in shades of turquoise and seafoam, some catching the light with delicate beading and subtle fringe detail cascading down from the bust. The floor-length silhouette skims the body before flaring gently at the hem, giving the whole look a soft, underwater kind of movement, unlike anything else currently making the rounds on red carpets this season.

Lili Reinhart and Tom Bateman at "The Love Hypothesis" premiere in London

She wore her blonde hair down in loose, side-swept waves, letting the color of the dress take center stage, and finished the look with delicate drop earrings that echoed the gown's turquoise tones. Bateman, meanwhile, kept it simple in a navy double-breasted suit worn open over a white shirt, no tie, styled with dark stubble and tousled curls for an easy, unfussy match to Reinhart's more elaborate gown.

Tom Bateman in navy suit with Lili Reinhart at Love Hypothesis premiere

The London stop comes right on the heels of the film's Brazil press tour, where Reinhart also turned heads in a separate rare vintage Ungaro two-piece featuring the house's signature butterfly print. "The Love Hypothesis" follows Olive Smith (Reinhart), a PhD candidate who fakes a relationship with intimidating professor Adam Carlsen (Bateman) to convince her best friend she's moved on, only for the arrangement to blur into something real. Directed by Claire Scanlon, the film also stars Rachel Marsh, Jaboukie Young-White and Arty Froushan, and premieres on Prime Video September 23.