The letter, written by international charity ONE as part of its Poverty is Sexist campaign, calls for gender equality and for politicians to take action.
ONE released the letter on International Women's Day in February, urging world leaders to shed light on how poverty affects millions of women.
Celebrities who have added their names to the letter include Emilia Clarke, Chadwick Boseman, Reynolds' wife and actress Blake Lively, Meryl Streep, U2 frontman and co-founder of ONE Bono, Reese Witherspoon, Daisy Ridley, Mindy Kaling, Natalie Portman, Robin Wright and Zendaya, among many others.
"For 130 million girls without an education. For 1 billion women without access to a bank account. For 39,000 girls who became child brides today. For women everywhere paid less than a man for the same work," the letter says.
"Poverty is sexist. And we won't stand by while the poorest women are overlooked," it continues. "We won't stop until there is justice for women and girls everywhere. Because none of us are equal until all of us are equal."