Sheldon McClellan and Myck Kabongo each scored 13 as Texas (7-2) won its fifth straight game.
Eddie Rios had 11 points for Texas State.
Texas is 40-5 all-time against Texas State and has not lost since 1937.
Texas scored 14 straight points in the first half as part of a 26-4 run and held Texas State (6-3) without a field goal for over 14 minutes to go up 29-10.
Texas State shot 14 percent from the field in the opening half (3 of 22) and trailed 38-19 at halftime.
Texas opened the second half with six 3-pointers, including three from Lewis, to increase its advantage to 56-26 within four minutes.