Greta Tintin Eleonora Ernman Thunberg (Swedish: [ˈɡreːta ˈtʉːnbærj] ^ⓘ; born 3 January 2003) is a Swedish activist best known for pressuring governments to address climate change and social issues. She gained global attention in 2018, at age 15, after starting a solo school strike outside the Swedish parliament, which inspired the worldwide Fridays for Future movement. She has since broadened her focus to include human rights and global justice, voicing support for Ukraine, Palestine, Armenia, and Western Sahara. In 2025, Thunberg twice joined a humanitarian flotilla bound for the Gaza Strip.

Thunberg has been credited with sparking the "Greta effect", influencing environmental awareness and youth engagement worldwide. She has received numerous honors, including inclusion in Time's 100 Most Influential People and being named Person of the Year in 2019.

Greta Tintin Eleonora Ernman Thunberg was born on 3 January 2003, in Stockholm, Sweden, to opera singer Malena Ernman and actor Svante Thunberg. Her paternal grandfather was actor and director Olof Thunberg. As explained by The Week, "with a thespian father" and singer mother, "it is perhaps unsurprising that [Thunberg] has a slightly unusual name.... Thunberg shares her second name with the adventuring creation of Belgian cartoonist Georges Remi, better known as Hergé." She has a younger sister, Beata.

Thunberg says she first heard about climate change in 2011, when she was eight years old, and could not understand why so little was being done about it. The situation depressed her, and as a result, at the age of 11, she largely stopped talking, severely restricted her eating, and lost ten kilograms (22 lb) in two months. Eventually, she was diagnosed with Asperger syndrome, obsessive–compulsive disorder (OCD), and selective mutism. In one of her first speeches demanding climate action, Thunberg described her selective mutism as meaning she "only speaks when necessary". She struggled with depression for almost four years before she began her school strike campaign. When she started protesting, her parents did not support her activism. Her father said he did not like her missing school but added: "[We] respect that she wants to make a stand. She can either sit at home and be really unhappy, or protest and be happy."

I was diagnosed with Asperger's syndrome, OCD and selective mutism. That basically means I only speak when I think it's necessary. Now is one of those moments.

Thunberg's diagnosis of Asperger's syndrome was made public nationwide in Sweden by her mother in May 2015, in order to help families in similar situations. While acknowledging that Asperger's "has limited [her] before", Thunberg views her diagnosis positively and has described it as a "superpower". She was later described as not only the best-known climate change activist, but also the best-known autism rights activist. In 2021, Thunberg said that many people in the Fridays for Future movement are autistic, and very inclusive and welcoming. She thinks that the reason so many autistic people become climate activists is that they cannot look away, and have to tell the truth as they see it: "I know lots of people who have been depressed, and then they have joined the climate movement or Fridays for Future and have found a purpose in life and found friendship and a community that they are welcome in." She considers the best things that have resulted from her activism to be friendships and happiness.

For about two years, Thunberg challenged her parents to lower the family's carbon footprint and overall impact on the environment by becoming vegan, upcycling, and giving up flying. She has said she showed them graphs and data, but when that did not work, she warned her family that they were stealing her future. Giving up flying in part meant her mother had to abandon international ventures in her opera career. Interviewed in December 2019 by the BBC, her father said: "To be honest, [her mother] didn't do it to save the climate. She did it to save her child, because she saw how much it meant to her, and then, when she did that, she saw how much [Greta] grew from that, how much energy she got from it." Thunberg credits her parents' eventual response and lifestyle changes with giving her hope and belief that she could make a difference. Asked in September 2021 whether she felt guilty about ending her mother's international career, she was surprised by the question: "It was her choice. I didn't make her do anything. I just provided her with the information to base her decision on." The family's story is recounted in the 2018 book Scenes from the Heart, updated in 2020 as Our House Is on Fire: Scenes of a Family and a Planet in Crisis, with contributions from the girls, and the whole family credited as authors.

In August 2018, Thunberg began the school climate strikes and public speeches for which she has become an internationally recognized climate activist. In an interview with Amy Goodman of Democracy Now!, she said she got the idea of a climate strike after school shootings in the United States in February 2018 led several youths to refuse to return to school. These teen activists at Marjory Stoneman Douglas High School in Parkland, Florida, went on to organize the March for Our Lives in support of greater gun control. In May 2018, Thunberg won a climate change essay competition held by Swedish newspaper Svenska Dagbladet. In part, she wrote: "I want to feel safe. How can I feel safe when I know we are in the greatest crisis in human history?"