The 30-year-old tells Michelle: "I am proud to have my daughter grow up in a world where she has people like you to look up to."
In the hand-written letter, posted on her new website, she calls Mrs Obama "the ultimate example of a truly strong African-American woman."
She also praises Barack Obama's wife for being a caring mother, and a loving wife, while at the same carrying out her duties as first lady.
"She builds and nurtures her family, while also looking out for so many millions in so many ways," writes B.
The Girls Run the World singer has long been a supporter of the first family – she even provided the soundtrack for their first dance at the Inauguration Ball, singing At Last.
And she showed off her bootylicious credentials when she led a school dance session as part of Michelle's Lets Move campaign.
Most recently, the star attended the first lady's fundraiser to bolster the US president's re-election campaign.
Beyonce's letter featured on her revamped website, on which she has also posted a series of intimate family snaps.
She can be seen showing off her bump during her pregnancy and enjoying exotic holidays with husband Jay-Z and sister Solange.
There are no snaps of baby Blue Ivy, however.