Davis, who accepted the USC job Jan. 7, told USC coach Steve Sarkisian he was leaving for family reasons.
"We wish Bo and his family the best of luck. He is a good coach," Sarkisian said in a statement. "We will move forward and find a great coach for our defensive line."
Davis coached defensive tackles at Texas from 2011-13 and prior to that spent eight of the previous nine years on Nick Saban's staff at Alabama, LSU and the Miami Dolphins. In all, he spent four seasons at Alabama, helping the Crimson Tide to a win in the 2010 BCS National Championship Game over Texas.
The 42-year-old Davis played nose guard at LSU, earning All-SEC second team honors as a senior in 1992.
Davis replaces former Alabama defensive line coach Chris Rumph, who left for the same position at Texas.
Davis is credited for recruiting current Texas commitments Jermaine Roberts, a four-star cornerback from New Orleans, and Lorenzo Joe, a four-star athlete from Abilene, Texas, among others. During his time in Austin, the Longhorns finished fifth, third and 16th in the ESPN class rankings.