Lawyers for Phillip Garrido argued that he is not mentally competent for trial on 18 counts of kidnapping, rape and false imprisonment plus special circumstance allegations.
A jury had been set to hear arguments on the competency issue, but both sides said in court Thursday they agreed instead to let the judge decide the matter.
El Dorado County Superior Court Judge Douglas Phimister quickly issued his ruling, based on numerous psychiatric reports.
A trial date was not immediately set. Garrido is due back in court at the end of the month.
Garrido, a convicted rapist, and his wife, Nancy, are both charged in the Dugard's abduction from a South Lake Tahoe bus stop in 1991.