The unseeded Polish teenager overpowered the fourth-seeded 21-year-old Russian-born American 6-4, 6-1 in their women's singles championship match in Paris.
Świątek did not drop a set on her way to the title.
She went into the tournament ranked world No. 54 by the Women's Tennis Association (WTA), but is set to climb to No. 17 when the rankings are updated on Monday.
Poland's Iga Świątek celebrates with the trophy after winning the French Open on Saturday, Oct. 10, 2020. Photo: EPA/YOAN VALAT
The victory at Roland Garros marked a new career milestone for the Polish 19-year-old, who was instantly showered with praise at home and abroad.
Polish President Andrzej Duda said on Twitter: "Great thanks and congratulations for Iga Świątek! A historic day for Poland, for Polish sport and for Polish tennis. Bravo!"
Prime Minister Mateusz Morawiecki tweeted: "Congratulations to Iga Świątek. Excellent performance in the Roland Garros final and first Grand Slam championship for Poland!"
In addition to lifting the French Open singles trophy, Świątek reached the semifinals of this year's women’s doubles competition in Paris together with Nicole Melichar of the United States.
The two failed to make the final after losing 6-7, 6-1, 4-6 to the pairing of Chilean Alexa Guarachi and American Desirae Krawczyk on Friday.
A previous high point in Świątek's career was when she reached the final of the WTA Samsung Open tournament in Lugano, Switzerland, in April last year.
In July 2018, the Pole won the girls' singles title at the Wimbledon championships in London.
Later that year, she won a gold medal for her country at the Youth Summer Olympic Games in Buenos Aires.
In December 2018, she was hailed as rookie of the year by public broadcaster Polish Radio.
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Source: Eurosport, PAP