Topping his all-black ensemble with a baseball cap and camouflage jacket, the 44-year-old actor walked alongside Allen, who was also wearing an all-black look in addition to a Chanel bag with some big kitschy labels on the side.
The low-key pair were also joined by the Stranger Things star's mother and father, who were all smiles for their night out in Manhattan.
Prior to heading to the after-party, Allen made sure fans knew she was watching Harbour's SNL performance. She shared two images from a particular sketch in which the leading man portrays a hilarious, tank top-wearing SoulCycle instructor. And in the second image, the 34-year-old singer zeroed on his large bicep, writing alongside his muscles, "Mine."
Allen wasn't the only SNL viewer who took notice of Harbour's impressive biceps in the aforementioned sketch. After it aired, Twitter was flooded with incredulous comments from fans who were very excited about his bulging muscles.
Rumors have been circulating for a few months now that the Stranger Things actor and the British singer-songwriter are dating, though neither have publicly commented on the status of their relationship. Before the SNL soiree, they were spotted together over the summer in London, where he was filming Black Widow.
Harbour was previously linked to Fantastic Beasts star Alison Sudol, while Allen's three-year relationship with rapper Meridian Dan ended in December 2018.
At the premiere for season three of Stranger Things, Harbour shared his reaction to the shocking finale's post-credits scene. Check it out below.