Arkansas inmate Gregory Holt.

WASHINGTON — An Arkansas policy prohibiting inmates from having beards violated the religious rights of a prisoner who had wanted to grow one in accordance with his Muslim beliefs, a unanimous U.S. Supreme Court ruled on Tuesday.

The justices, on a 9-0 vote in a closely watched case involving prisoner Gregory Holt, rejected the state’s reasoning that the policy was needed for security reasons to prevent inmates from concealing contraband.

Holt, who wanted to grow a half-inch beard (1.3 cm), is serving a life sentence for burglary and domestic battery at the Varner Supermax prison, according to the Arkansas Department of Correction. In 2005, he pleaded guilty to separate charges of threatening the daughters of then-President George W. Bush.

Holt, without any legal representation at the time, persuaded the court to hear his case by filing a handwritten petition.

Justice Samuel Alito, writing on behalf of the court, said the state already searches clothing and hair and had not given a valid reason why it could not also search beards.

Alito wrote that the prison’s “interest in eliminating contraband cannot sustain its refusal to allow petitioner to grow a half-inch beard.”

Holt said the state’s prison grooming policy prohibiting inmates from having facial hair other than a “neatly trimmed mustache” violated his religious rights under a 2000 federal law called the Religious Land Use and Institutionalized Persons Act.

His lawyers noted that more than 40 states and the federal government allow prison inmates to have similar beards.