Sanna Marin, 34, was photographed wearing a smart black trouser suit with a plunging neckline for the cover of fashion magazine Trendi.
Politicians and members of the public have been divided in their opinions, with some calling the image “inappropriate” and others praising Ms Marin.
Supporters of the Finnish leader shared similar images of themselves on social media under the hashtag #imwithsanna, while some posted topless photos of male world leaders, including Russian president Vladimir Putin.
Finnish actress Kiti Kokkonen and singer Hanna Pakarinen also posted images of themselves in the same pose to show their support for the prime minister.
“Everyone should be allowed to live in such a way that their personality, dignity and professional skills are not determined by grabbing irrelevant flower stalks,” Ms Kokkonen wrote on Instagram.
Rosianne Cutajar, Malta’s junior minister for equality and reforms, retweeted the image of Ms Marin, writing: “Regardless of our work and our roles in society and politics, all too often we are judged by our appearance. Sexist and misogynistic attacks continue unchecked. When will this shaming stop?”
While Latvian welfare minister Ramona Petraviča said politicians “must be judged by their achievements, not their looks”.
Aki Pyysing, a Finnish entrepreneur, mocked Ms Marin’s cleavage and claimed she was “attention-seeking” in a blog post shared by two MPs from the opposition centre-right National Coalition party.
However the MPs, Elina Lepomaki and Juhana Vartianen, later apologised and deleted their tweets.
Mari Karsikas, Trendi’s editor-in-chief, suggested the “furore” stemmed from misogyny, writing: “Many did not see her chest at all — they saw Sanna Marin dressed in a trendy black trouser suit”, according to The Times.
Social Democrat Ms Marin became the world’s youngest leader, the youngest prime minister in Finland's history and only its third ever female premier when she was sworn into office in December.
She was born and raised in Helsinki by her mother and her mother’s female partner, in what she described as a “rainbow family”.
She was the first person in her family to graduate both high school and university, where she obtained a master's degree in administrative sciences in 2017.