Duggan becomes the first English player to leave home shores for the Nou Camp since Gary Lineker’s move in 1986.
The 25-year-old leaves Women’s Super League One champions City for an undisclosed fee after spending four years with the club.
Duggan has 15 goals in 47 appearances for England and will represent the Three Lions once again at the European Championships later this year.
“FC Barcelona are known all over the world for playing attractive football and I naturally wanted to be part of that,” Duggan said.
“I think my style of play will suit the team and I’m honoured to be part of the club.
“My new coach Fran Sanchez has told me that their ambition is to win the Women’s Champions League and the club have made that very clear with their signings over the years.
“I’ve had a lot of success in England and I’ve really enjoyed my time with Manchester City but I wanted to challenge myself even further by playing abroad. I’m also looking forward to learning Spanish and living in the city.”
Duggan helped City to the Super League title last year before being part of the squad that won the the Women’s FA Cup earlier this year.