Anna van der Breggen of the Netherlands won the nine-stage race, which ended on Saturday, while Italy’s Elisa Longo Borghini placed third overall.
"I cannot find the right words to describe how happy I am right now," Poland's Niewiadoma, who rides for the Canyon-SRAM team, said in an Instagram post celebrating her success.
The event was held ahead of this week’s world road cycling championships in Imola, Italy.
The 2020 UCI Road World Championships will be held from September 24 to 27 after a delay due to the coronavirus pandemic.
The women’s time trial is scheduled for Thursday and the road race is due to take place on Saturday.
The 25-year-old Polish rider last month took the bronze in the women’s elite road race at the European championships in Plouay, France.
The Pole last year hit a career high by winning the one-day Amstel Gold Race in the Netherlands.
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Source: IAR