Greta Thunberg, a climate activist, was fined for defying a police order to leave a climate protest in Malmö, Sweden, last month.
Despite his denial of the accusation, 20-year-old Thunberg was found guilty and sentenced to pay 2,500 Swedish krona ($240; £187).
She took part in another protest a few hours later, and she and five other people were taken away by police.
She was a member of two protesting groups that blocked the entrance to Malmö harbor for oil trucks.
She was charged in June for resisting a police request to leave.
She told the court in Malmö on Monday, "I believe that we are in an emergency that threatens life, health, and property," adding that "countless people" were in danger.
She said, "It's true that I was there on that day and that I was given an order that I disregarded, but I want to deny the crime.".
June's rally was planned by an environmental activist group. Retrace your steps backwards. (Reclaim the Future) attempted to block the Malmö harbor's entrance and exit in a show of opposition to the burning of fossil fuels.
"We make the conscious decision to physically obstruct the fossil fuel infrastructure rather than just observe. We are reclaiming the future," Thunberg said in an Instagram post at the time.
Reclaim the Future claims that it is still committed to opposing the industry.
The fossil fuels industry stands in the way of our right to live, according to group spokesperson Irma Kjellstrom, who said, "If the court chooses to see our action as a crime it may do so.".
Activities like burning fossil fuels have pumped greenhouse gases into the atmosphere. When compared to the late 19th century, the world has warmed by about 1 point 1C.
Fossil fuel companies have come under fire from environmental activists. According to a large portion of the oil and gas industry, ongoing production is required to meet the world's energy demands.