Who can decide the constitutionality of a state law?

The constitutionality of a state law can be decided by the state courts, particularly the state supreme court, which has the authority to interpret state laws and the state constitution. Additionally, federal courts, including the U.S. Supreme Court, can also rule on the constitutionality of state laws if they involve federal constitutional issues. Ultimately, if a state law conflicts with the U.S. Constitution, it is subject to being invalidated by federal courts.