The bizarre cloud floated in the sky over ski resorts Åre and Duved on Thursday, prompting some to speculate that Martians had arrived for a day on the slopes.
Many rushed to take pictures of the unusual phenomenon and posted them on social media.
"I thought it was pretty cool," one of them, Elvira Kuper, told broadcaster SVT.
Experts explained it was a so-called lenticular cloud, which forms over mountain peaks when the air is forced to rise as it hits the hillside. As it cools it condenses into a cloud.
Thanks to their peculiar shape, they are often mistaken for UFOs.
Here are some of the best pictures of the cloud: