The singer, 20, first stepped out with the Austrian in Los Angeles this past November after meeting her through CAA agent Jeff Azoff, reported Us magazine.
"Harry has been getting more serious with Nadine," a source said.
Leopold and Styles are apparently spending lots of time together.
"He planned a few activities for her birthday," the source added.
Prior to dating Leopold, the British star was linked to Kendall Jenner, Cara Delevingne, and Taylor Swift, whose 1989 album is chock-full of songs inspired by their 2012 romance.