State Highway Spur 421 (Spur 421) is a state highway spur in the San Antonio, Texas, USA metropolitan area.

[1] Spur 421 begins at an intersection with I-10 north of Downtown San Antonio.

[1][2] Spur 421 is a former route of SH 16, and was designated (along with Spur 422) on November 16, 1965, after SH 16 was rerouted around the west side of San Antonio along the Loop 410 freeway.

[1] The section of Spur 421 south of Callaghan Road was proposed for decommissioning in 2014 (Spur 421 would have also replaced the section of SH 16 north to Eckhert Road as well) as part of TxDOT's San Antonio turnback proposal, which would have turned back over 129 miles of roads to the city of San Antonio, but the city of San Antonio rejected that proposal.

[3] The entire route is in Bexar County.