The 27-year-old guided Wolfsburg to the Champions League final last month and scored a total of 103 goals for the German side in 113 appearances over the past three-and-a-half years.
"I'm excited to just play for such a big club, to play with so many amazing players and to also be playing in the league," said Harder. "The English league, it's so exciting at the moment and I'm really looking forward to it.'
Harder will join forces with Australian international Sam Kerr in West London, while title rivals Manchester City have reinforced for the 2020/21 season with the signings of American World Cup winners Sam Mewis and Rose Lavelle.
"Pernille is one of the best players in the world. Her record at Wolfsburg and Linkopings, as well as internationally with Denmark, shows how valuable she has been to her teams," said Chelsea manager Emma Hayes.
"She is a fantastic team player who wants to take her game to the next level. She chose Chelsea as the club that can push her to that level. She can play for any team in the world, but she chose us.
"She chose the players, the staff, the environment, the playing style. You can't get a bigger compliment than that from one of the best players in the world."
Chelsea were awarded the 2019/20 Women's Super League title on points-per-game ahead of City after the season was curtailed due to the coronavirus pandemic.
They begin their title defence away to Manchester United on Sunday.
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