The pop star took to Instagram alongside brother and music producer Finneas Tuesday, where they encouraged their U.S. followers to register to vote ... and endorsed the Vice President of the United States in her bid for the White House.

Billie said ... "We are voting for Kamala Harris and Tim Walz because they are fighting to protect our reproductive freedom, our planet, and our Democracy."

BE encouraged her fans to "vote like your lives depend on it," saying she and Finneas plan to vote early this year.

Finneas chimed in with his own 2 cents, encouraging voters not to let "extremists control our lives, our freedoms, and our future."

He continued ... "The only way to stop them and the dangerous Project 2025 agenda is to vote -- and elect Kamala Harris."

Billie and Finneas' high-profile endorsement comes a week after Harris faced off with Donald Trump in their first presidential debate. Following the debate, Taylor Swift posted her endorsement of the Harris-Walz ticket, dispelling any mystery about her vote this November.

Taylor made it clear she was voting for Harris as "she fights for the rights and causes I believe need a warrior" ... citing reproductive freedoms, LGBTQ+ rights and more as reasons for her support.

While Billie and Taylor are on the same page here, fans speculated the 2 stars were beefin' earlier this year. Billie called out artists for not considering the environmental impact of producing multiple vinyl versions of the same album ... with many Swifties assuming Billie was talking about Taylor.

Billie later clarified she never singled out one artist ... but fans still felt there was awkwardness between the 2 singers.