Earlier this week, Ms Marin found herself at the centre of controversy after posing for a photo wearing a black blazer with a plunging neckline and no blouse in a magazine spread.
The 34-year-old appeared in the country's leading fashion publication Trendi over the weekend as its October cover star.
However, the politician's outfit choice in one of the images drew criticism from some social media users and members of the opposition, who suggested her outfit was “inappropriate” for someone in her position.
One person said the outfit “eroded” Ms Marin’s credibility, while another accused the prime minister of using the photo as a “tasteles" stunt.
Elsewhere, a Finnish entrepreneur wrote a blog post in which he criticised Ms Marin’s policies and also mocked the size of her breasts in the image, accusing her of “attention-seeking”.
The article was later shared by two MPs, including Elina Lepomaki, who is a member of the National Coalition party, the main force in opposition.
In response to the backlash, supporters of the prime minister launched a social media campaign using the #ImWithSanna hashtag to share photos of themselves wearing similar outfits to Ms Marin while calling out critics for being sexist.
“When people run out of ways to criticise women based on substance, they go for appearance,” on person wrote alongside their own copycat image.
“A woman’s job is not to dress to please others. No one’s position or capabilities should ever be judged based on their looks - or their neckline.”
Another added: “Professionalism is not defined by the depth of a neckline.”
Trendi Magazine shared some of the #ImWithSanna images to its Instagram account, showing women of all ages posing wearing a plunging black blazer.
Mari Karsikas, Trendi’s editor-in-chief, wrote: “The furore clearly shows that what people saw in the pictures said a lot about the viewer. Many did not see her chest at all — they saw Sanna Marin dressed in a trendy black trouser suit.” She added that it was hard to interpret the negative comments as anything other than misogyny.
So far, the hashtag has more than 1,700 posts on Instagram.
As the campaign began to grow, Ms Lepomaki, one of the MP’s who shared the blog post about Ms Marin, apologised and deleted the tweet.
“My mistake. I shared it carelessly and did not read the entirety of the text, which got personal and mocked the prime minister’s appearance,” she wrote.
“I do not support such a culture of conversation. Please @MarinSanna I'm sorry.”
Ms Marin became Finland's prime minister in December 2019.
At the time she was the world's youngest national leader and promptly appointed three other women in their thirties to her cabinet.
In August she married her long-term partner Markus Raikkonen, with whom she shares a two-year-old daughter.
The couple have been together for 16 years after meeting when they were 18.