The State Border Security Assistance Act creates two new federal grant programs to help states and local governments with border security and immigration enforcement. The first program, the State Border Security Reinforcement Fund, provides $11 billion in grants through the Department of Homeland Security for activities like building border walls and fences, ground preparation, surveillance systems, and relocating undocumented immigrants from small towns. The second program, the State Criminal Alien Prosecution and Detention Fund, provides $3.5 billion in grants through the Department of Justice for states and localities to locate and apprehend undocumented immigrants, investigate crimes involving immigrants and human trafficking, prosecute cases, and cover detention and transportation costs. Both funds remain available through September 2034 and can reimburse states for eligible activities that began on or after January 20, 2021. The programs automatically expire on January 20, 2029, and any unspent money must be returned to the Treasury to help reduce the federal deficit.