Knowles v. Iowa, 525 U.S. 113 (1998), was a decision by the United States Supreme Court which ruled that the Fourth Amendment prohibits a police officer from further searching a vehicle which was stopped for a minor traffic offense once the officer has written a citation for the offense.

Knowles was then arrested and charged with violation of state laws dealing with controlled substances.

At the hearing on the motion to suppress, the police officer conceded that he had neither Knowles' consent nor probable cause to conduct the search.

Because Iowa Code § 321.485(1)(a) permits either an arrest or a citation when making a traffic stop, the Iowa Supreme Court has interpreted this provision as providing authority to officers to conduct a full-blown search of an automobile and driver in those cases where police elect not to make a custodial arrest.

No further evidence of excessive speed was going to be found either on the person of the offender or in the passenger compartment of the car.Because, given the type of stop, there were no grounds for the officer to believe that his safety was in jeopardy, and thus had no probable cause to perform a search without consent of the driver.