The district—which stretches from Travis County along the I-35 corridor through Hays, Comal and into downtown San Antonio in Bexar County—has no incumbent.
The winner of the Republican runoff will face Democrat Greg Casar in the general election.
The two candidates received the most votes in the March Republican Primary in a ten-way race, with no candidate receiving a majority of the votes.
Results are unofficial until canvassed.