The "Legally Blonde" actress got personal with her followers on Instagram, beginning: "I woke up this morning thinking about a moment from my childhood."
"In the 3rd grade our teacher asked us what we wanted to be when we grew up, I raised my hand and said, 'I want to be the first female President of the United States of America,'" she recalled while posting a photo of her at that age.
"Some of the kids laughed, but my teacher looked me in the eye and said, 'I will be the first person to vote for you, Reese,'" the 44-year-old continued.
Witherspoon said her thoughts are with women all across the country -- "especially the young Black and Brown girls of our nation," she specified.
"Now you know that YOU too can work hard, dedicate your life to service and show up every day to help others, and YOU can RISE to become a Vice Presidential nominee in the United States of America," she added. "Today is a truly historic day!"
To end her post, she added the hashtag "#WeHaveHerBack" along with Harris' name.
Harris, 55, is currently a Democratic senator from California and ran against Biden, 77, for the presidential nomination before joining him on the ticket.
If elected, Harris, who is Black and Indian, would not only become the first female vice president of the United States, but also the first person of color to hold the position.