Lively shared a photo of members from the United States Army parading down a street while carrying a variety of LGBT flags, including one symbolizing the transgender community.
"THIS is America," she captioned it, tagging the American Civil Liberties Union (ACLU). "WE the people. ALL THE PEOPLE. Today I stand with our fellow citizens. No human being is a burden."
"I believe in equality. In freedom. In opportunity. In kindness, acceptance, and fairness," she continued. "I believe in this country, so I, and many others, sign the petition linked in the bio. Please join..??????"
The link takes fans to the "Allow Transgender People Into the U.S. Military" petition on change.org. As of Thursday afternoon, the petition already had 184,600+ supporters, with a goal to reach 200,000.
As ET previously reported, Trump tweeted that after consultation with his "Generals and military experts, the United States Government will not accept or allow transgender individuals to serve in any capacity in the U.S. Military."
"Our military must be focused on decisive and overwhelming victory and cannot be burdened with the tremendous medical costs and disruption that transgender in the military would entail," he added. "Thank you."
The proposed policy is a reversal of the Obama administration's decision that would have allowed transgender individuals to serve.
Click HERE to read more reactions from A-listers like Zelda Williams, Demi Lovato, Andy Cohen, Ellen DeGeneres and Seth Rogen.