The social network's chief operating officer, who has been with the company for four years, joins a board that now consists of herself and seven men.
"Sheryl has been my partner in running Facebook and has been central to our growth and success over the years," said Facebook CEO Mark Zuckerberg.
"Her understanding of our mission and long-term opportunity, and her experience both at Facebook and on public company boards, makes her a natural fit for our board."
Sandberg, who has spoken out against how male-dominated the technology industry is, played a central role in guiding Facebook towards its stock market flotation last month.
The executive joined the company following a stint with Google, where she helped the search engine giant's advertising business become the financial powerhouse it is today.