Taylor Swift, one of America’s most celebrated pop-culture icons with an enormous following across the world, endorsed Vice President Kamala Harris late Tuesday in the immediate aftermath of the presidential debate.

The endorsement by Ms. Swift, delivered mere minutes after Ms. Harris and former President Donald J. Trump stepped off the debate stage in Philadelphia, offers Ms. Harris an unrivaled validator in the world of celebrity.

“Like many of you, I watched the debate tonight,” Ms. Swift wrote on Instagram to her 283 million followers. “I will be casting my vote for Kamala Harris and Tim Walz in the 2024 Presidential Election. I’m voting for @kamalaharris because she fights for the rights and causes I believe need a warrior to champion them."

She signed her post as “Childless Cat Lady,” a reference to comments made by Mr. Trump’s running mate, Senator JD Vance of Ohio, about women without children. The photo that accompanied her post showed her holding a furry feline, Benjamin Button, her pet Ragdoll.

Ms. Swift’s endorsement was much anticipated for Democrats. She has embraced a more political posture during the last several years and become more outspoken on issues such as abortion access. But its precise timing was something of a surprise.

Ms. Swift is one of the few celebrities with broad appeal and the ability to cut through a crowded media environment. Her romance with Travis Kelce, the star tight end for the Kansas City Chiefs, has captivated the world of football and culture, and she is in the final stages of a head-spinning international tour that has sold out stadiums across the globe.

But Karoline Leavitt, a spokeswoman for the Trump campaign, dismissed the endorsement as “more evidence that the Democrat party has become the party of the wealthy elite.”

Jess Bidgood, Michael M. Grynbaum and Michael Goldcontributed reporting.