WASHINGTON—The Supreme Court unanimously held Thursday that human genes cannot be patented, even when isolated from the body, a ruling expected to quickly expand access to genetic testing while potentially allowing inventors to retain rights to artificially created DNA.

The decision marked the latest step in the court's decadelong march to toughen the requirements for patents. The justices repeatedly have declared that 21st-century innovation depends less on locking up intellectual-property rights than on expanding access...