Uvalde County officials confirmed to KABB/WOAI-TV that at least 12 people were killed and three were injured in the collision on Highway 83 near Garner State Park in the town of Concan.
The accident was reported around 2 p.m. local time.
Rick Ballesteros of Uvalde, who came upon the scene of the accident, told KABB/WOAI-TV the church bus was carrying older passengers.
"It was a really gruesome scene," he said in a phone interview.
The First Baptist New Braunfels church asked for those to "please be in prayer for all involved" in a statement on its Facebook page.