Kraft Foods boss Irene Rosenfeld came second in the magazine's annual list.
Media mogul Oprah Winfrey, German Chancellor Angela Merkel and US Secretary of State Hillary Clinton completed the top five.
Singers Lady Gaga and Beyonce Knowles made the top 10 while Queen Elizabeth II appears at number 41.
Forbes said this year's list was based "less on traditional titles and roles and more on creative influence and entrepreneurship".
Mrs Obama had "given a new generation of girls and women around the world a role model", the magazine said.
Thursday, October 7, 2010
I just do not get this at all. Michelle might be a nice lady but what gives her top ranking in front of all these accomplished women. I don't know, not being proud of your country, starting a garden on the White House grounds, and telling people what they should and should not eat does not put her on the top of most powerful, but maybe the most opinionated.