Kimmel showed Portman on Monday a recent shirtless photo of Hemsworth working out by lifting a giant tire. Portman is currently in Australia and is also training for the superhero epic.
"It's otherworldly," Portman said about Hemsworth's muscles before she poked fun at the actor's lack of a tan.
"I'm so unversed in what muscles do and how they get like that. I was like does the blood drain out of you when you use your muscles because it looks so white doesn't it? But I feel like it's a t-shirt tan," the actress continued about Hemsworth's pale skin.
Portman suggested that Hemsworth get a spray tan to help with the muscle pictures.
"He's looking good, it's a lot of pressure. I'm going to look like his little grandma next to him," she said.
Thor: Love and Thunder, from director Taika Waititi, is set to hit theaters on Feb. 11, 2022. Portman's character Jane Foster will gain superpowers in the sequel and become The Mighty Thor.