In criminal law, police perjury, sometimes informally called "testilying", is the act of a police officer knowingly giving false testimony. It is typically used in a criminal trial to "make the case" against defendants believed by the police to be guilty when irregularities during the suspects' arrest or search threaten to result in their acquittal. It also can be extended to encompass substantive misstatements of fact to convict those whom the police believe to be guilty, procedural misstatements to "justify" a search and seizure, or even the inclusion of statements to frame an innocent citizen. More generically, it has been said to be "[l]ying under oath, especially by a police officer, to help get a conviction."

When police lie under oath, innocent people can be convicted and jailed; hundreds of convictions have been set aside as a result of such police misconduct. Some sources say that it is both a police and a prosecutorial problem and that it is a systemic response to the fruit of the poisonous tree doctrine, which was recognized in the US Supreme Court decision Mapp v. Ohio. Other authors have drawn a connection between perjury and an increased emphasis on the number of arrests and convictions made.

The extent of the practice is debated; journalists, activists, and defense attorneys have exposed numerous instances of false testimony, but police officers and police unions, while acknowledging its occurrence, deny that it is widespread or systemic. The defense attorney Alan Dershowitz argued, in the New York Times and before a congressional hearing, that police perjury is commonplace:

As I read about the disbelief expressed by some prosecutors.... I thought of Claude Rains's classic response, in Casablanca on being told there was gambling in Rick's place: "I'm shocked—shocked." For anyone who has practiced criminal law in the state or Federal courts, the disclosures about rampant police perjury cannot possibly come as a surprise. "Testilying"—as the police call it—has long been an open secret among prosecutors, defense lawyers, and judges.

In 1995, the Boston Globe reported that New York Police Commissioner William J. Bratton had created a furor by saying that he agreed with most of Dershowitz's statement. The Globe quoted Richard Bradley, then-president of the Boston Police Patrolmen's Association: "I find it incredible that he would say that. Every day all over the country, police officers are testifying. Everyone realizes they are testifying under oath. If this was this much a problem, it would have come to light over the years." Bradley said that in 27 years on the Boston force he had never encountered the practice.

In a 1996 article in the Los Angeles Times, "Has the Drug War Created an Officer Liars' Club?", Joseph D. McNamara, the chief of police of San Jose, said, "Not many people took defense attorney Alan M. Dershowitz seriously when he charged that Los Angeles cops are taught to lie at the birth of their careers at the Police Academy. But as someone who spent 35 years wearing a police uniform, I've come to believe that hundreds of thousands of law-enforcement officers commit felony perjury every year testifying about drug arrests." He also noted, "Within the last few years, police departments in Los Angeles, Boston, New Orleans, San Francisco, Denver, New York and in other large cities have suffered scandals involving police personnel lying under oath about drug evidence."

In 2011, after finding a former police detective, Jason Arbeeny, guilty of official misconduct for planting drugs on a suspect, Justice Gustin L. Reichbach of the New York Supreme Court wrote that he "thought [he] was not naïve, but even this court was shocked, not only by the seeming pervasive scope of misconduct but even more distressingly by the seeming casualness by which such conduct is employed." Arbeeny was then sentenced to five years of probation and 300 hours of community service. Also in 2011, a former San Francisco Police Commissioner, Peter Keane, wrote that lying under oath was a "routine practice" for narcotics officers.

In 2019, the Foreign Intelligence Surveillance Court called out the Federal Bureau of Investigation and the National Security Division of the US Department of Justice for dishonesty in applications for continuance of a wiretap of Carter Page, saying that it called into question the reliability of other evidence submitted by the FBI. The practice was documented in a previous report released by the Department of Justice's Inspector General, Michael Horowitz, and was cited by the court.