The 26-year-old American singer showed her support for the 32-year-old Scottish DJ by enlisting several friends, including brother Austin Swift and model Karlie Kloss, to attend Harris' show at Omnia Nightclub in Caesars Palace.
Swift was spotted singing and dancing with her sibling, Kloss, actress Serayah and singer Todrick Hall. The group shared videos on social media, including a clip of Swift and Hall dancing to Rihanna song "We Found Love," which Harris wrote and produced.
"Last night was epic! Thank you @taylorswift for being the sweetest human on earth! Such a freakin blast!" Hall captioned the video on Instagram.
"Taylor wasn't on her phone and was just living in the moment," a source told E! News. "Toddy danced like there was no tomorrow just like Taylor did."
Swift and Harris reportedly hit it off at the 2015 Brit Awards and celebrated their one-year anniversary as a couple this month. Harris gave the singer a gold locket to mark the occasion, and the pair later embarked on a romantic getaway.
"They both had time off from their insanely busy schedules this past year and wanted to go somewhere away from it all to chill out," an insider told E!. "This was also part of their anniversary presents to each other."