A motion to suppress evidence is a formal, legal tool that an attorney or pro se litigant can use in a trial. It is a legal request submitted to the judge of the case, requesting that certain evidence be excluded from consideration in determining the outcome of a trial. A motion to suppress evidence, or motion to suppress, can be requested in a criminal or civil case.

In the United States, the legal system weighs each person's constitutional right to privacy against the public interest in prosecuting crimes and enforcing public and private laws. Various rules and Constitutional rights are in place to protect citizens' privacy and to prevent unlawful investigations. If a law enforcement organization violates one of these privacy laws and rules, evidence is considered to be improperly collected. When evidence is improperly obtained, such as through an illegal search, an attorney can file a motion to suppress that illegally obtained evidence. If the attorney can prove that the evidence was not obtained through lawful means, the judge must grant the motion to suppress evidence.

The laws designed to protect an individual's privacy include the Fourth Amendment, which protects against unreasonable search and seizure. The Fourth Amendment mandates that a person and his property cannot be searched without reasonable cause. Generally, a warrant is required, unless illegal items are in plain site or the police or law enforcement personnel witness a crime being committed.

The Fifth Amendment is also designed to protect a person's privacy. It guarantees a person the right to remain silent, and the right to avoid self incrimination. Under a Supreme Court case called Miranda v. Arizona, the police must inform a suspected criminal of this right — as well as of his right to an attorney — before questioning.

In the justice system, any additional evidence that police recover as the result of an illegal search or illegal questioning is also inadmissible. This evidence is considered "fruit of the poison tree." For example, if the police recover a gun only because the criminal told them where the gun is during the course of an illegally obtained confession, the gun is also improperly obtained evidence and can be suppressed.

When a motion to suppress evidence is granted, neither the judge nor the jury can make a decision about the case based on the suppressed evidence. The evidence is not allowed to be presented at trial at all. If the prosecutor cannot make a case without the illegally obtained evidence, the accused criminal may be acquitted.