Many evacuees were seen grabbing belongings from their homes before the law went into effect Thursday morning.
People who enter the zone will face fines up to $1,200 or possible jail time of up to 30 days.
About 80,000 people were evacuated after the March 11 tsunami and earthquake wrecked the Fukushima Dai-ichi plant's power and cooling systems. Many have returned to check on their houses or get things left behind.
Until now, police said they had no legal way to stop those residents from getting through.
Authorities said they'll arrange for limited visits so that people can get things from their homes. They will be required to go through radiation screening.
The no-go order was not due to any particular change in conditions inside the nuclear plant, which appear to have somewhat stabilized. Even under the best-case scenario, however, the plant's operator says it will take at least six months to bring its reactors safely into a cold shutdown.
Meanwhile, new data from Japan's National Police Agency showed that two-thirds of the victims identified in last month's earthquake and tsunami were elderly - and almost all of them drowned.
The agency said 65 percent of the 11,108 confirmed fatalities of known age were 60 or older. Another 1,899 victims were of unknown age.
Adding those who are still missing, the earthquake and resulting tsunami killed an estimated 27,500 people.
The Associated Press contributed to this report.