In legal proceedings, a star witness is a witness who provides major and crucial information in a criminal case. In some instances, a case may have several star witnesses, and their testimony may cinch a conviction or acquittal, especially when handled by a skilled lawyer. The testimony of star witnesses is often a topic of interest for the general public as well, especially in high profile cases, and in open courtrooms, there may be a number of bystanders present to listen to the testimony of a star witness.

A witness is someone who provides information in a court of law which is designed to shed light on a case. Juries and judges weigh the information provided by witnesses and submitted evidence to reach a decision in a trial, making the testimony of any witness important in the overall success of a trial. A star witness, however, may be the base upon which a case is built and presented in court, making his or her testimony crucial to the outcome of a trial.

All sorts of people can serve as star witnesses. In cases about corruption of government agencies, for example, a star witness might be a former employee who witnessed such corruption firsthand. Star witnesses may also have seen murders or other crimes, or they might be in possession of incriminating or exonerating information about a defendant. Many lawyers try to keep the identities of their star witnesses under wraps until the last minute, so that their testimony is a surprise, preventing the other side from preparing.

Typically, lawyers groom a star witness carefully before putting him or her on the stand. They may go over the testimony in a mock-courtroom, showing the witness what cross examination will feel like, and asking questions similar to those which might be used in court. The witness may be encouraged to practice vital testimony, and the legal team will typically also advise the witness on how to dress and act in court for the right impression.

When a star witness testifies in a high profile case, many news agencies comment on it, reporting the testimony if the trial is public and providing analysis of it. The performance of star witnesses will also be considered in any media postmortem of the trial, with analysts speculating on how the witness impacted the trial outcome. On occasion, the testimony of a star witness is also quite sensational, attracting a great deal of public speculation and comment.