The section requires officers to question the immigration status of people suspected of being in the country illegally, while officers are in the process of enforcing other laws.
U.S. District Court Judge Susan Bolton gave the go-ahead Tuesday for officers to proceed with that requirement.
That part of the law had been challenged for the past two years, ever since the law was signed by Governor Jan Brewer. Opponents contend the law will lead to systematic racial profiling and unreasonable detentions of Hispanics in the state.
The U.S. Supreme Court ruled in June that the law does not conflict with federal law. Judge Bolton said earlier in September she would not block the "show me your papers" provision, as it has been dubbed by opponents.
Tuesday's ruling gives the go-ahead for officers to being enforcing it in full immediately.
Some of the law's backers question how much cooperation they'll received from federal immigration agents. Those agents are required to pick up illegal immigrants and verify their status.
If federal agents decline to pick up illegal immigrants, local officers in some cases will likely have to let them go unless they're suspected of committing a crime that would require them to be brought to jail.
Arizona's law was passed in 2010 amid voter frustration with the state's role as the busiest illegal entry point into the country. Five states - Alabama, Georgia, Indiana, South Carolina and Utah - have adopted variations on Arizona's law.
The Associated Press contributed to this report.