The 25-year-old Aldcroft helped England win a third successive Six Nations title this year and secure back-to-back wins over New Zealand.
Dupont is the third Frenchman to clinch the award after Galthie won in 2012 and flanker Thierry Dusautoir in 2011 after leading France to the World Cup final.
He beat Australia flanker Michael Hooper, England lock Maro Itoje and Australia centre Samu Kerevi in the public voting.
Last year Dupont was voted the best men's Six Nations player — the first Frenchman to win the award. He stood out with his bursts of speed from the back of the scrum, his perceptive passing and support running.
Aldcroft captained England for the first time in an 89-0 win over the United States that extended the Red Roses' unbeaten test run to 18.
"It was an honour to be nominated in the first instance and I'm absolutely over the moon," she told the Six Nations website. "Individual awards are a reflection of a team effort and I'd like to dedicate this to all of the teammates, coaching and management staff that I work with."
She won the public vote ahead of England backrower Poppy Cleall, France halfback Laure Sansus and France winger Caroline Boujard.
Aldcroft is the fourth Englishwoman to win the award following winger Michaela Staniford (2012), backrower Sarah Hunter (2016) and centre-fullback Emily Scarratt two years ago.
"To follow in their footsteps is an amazing feeling," Aldcroft said.
Earlier in the week, All Blacks winger Will Jordan was named World Rugby's Breakthrough Player of the Year while England's Simon Middleton created history as the first coach of a women's team to win Coach of the Year.