That means prosecutors will be able to use evidence seized during the stop in Jeffs' criminal trial, now set to begin Sept. 10.
Utah Fifth District Judge James Shumate said Nevada Highway Patrol Trooper Eddie Dutchover had legal justification - an obscured temporary registration tag - to stop a Cadillac Escalade in which Jeffs was traveling last August.
A plate holder blocked the bottom portion of the temporary tag, making it unreadable while traveling at freeway speed in the dark, Shumate said.
He rejected Jeffs' attorneys arguments that Nevada law requiring clearly visible plates applies only to licenses and registrations issued by that state.
Under Nevada law, the trooper also had the right to get identifications from Isaac Jeffs, who was driving the vehicle, and Warren Jeffs, who was a passenger.
Warren Jeffs initially rebuffed the trooper, and that alone was reason enough to arrest him, the judge said.
Nevada law also sets a time frame for how long a person may be detained by a peace officer before being arrested or set free: 60 minutes.
Even though a computer system used by the Nevada Highway Patrol was inoperable at the time of the stop, the trooper identified Jeffs as a wanted fugitive within that window, the judge said.
Dutchover pulled over the 2007 Escalade at 9:04 p.m. on Aug. 28 on Interstate 15 north of Las Vegas. He became suspicious when Isaac Jeffs and Warren Jeffs gave conflicting accounts of their travel plans and became visibly nervous.
After observing a GPS unit and radar detector in the vehicle, the trooper said he began to suspect the men might be involved in drug trafficking or money laundering.
In a search of the vehicle, troopers found money, wigs, cell phones and documents addressed to "WSJ" or "the President." That was enough for one trooper to figure out the passenger was the polygamous sect leader wanted by the FBI.
By 9:52 p.m., Dutchover's sergeant and the FBI had been contacted and were en route to the scene, the ruling noted.
Jeffs was subsequently taken into custody. He has been incarcerated at the Purgatory Correctional Facility in Hurricane since Sept. 5 on two counts of being an accomplice to rape.
The charges are based on his role conducting a marriage between a 19-year-old man and a 14-year-old girl, who has testified she told Jeffs on several occasions she opposed the arranged union.