After a criminal trial ends in a conviction, the defendant can file a motion for a new trial. Courts grant these—though rarely—to correct significant errors that happened during trial or if substantial new evidence of innocence comes to light. A court may also grant a motion for a new trial if there has been a "miscarriage of justice." In short, new trial grants generally require that some kind of error prevented the defendant from receiving a fair trial.
Trial judges only grant motions for a new trial in very limited circumstances where no other remedy would do. State or federal law generally identifies the grounds on which a court may order a new trial.
Legal errors during trial comprise one basis for granting a new trial motion. An example is a judge having wrongly excluded evidence that would have made a difference to the outcome of the trial. An erroneous exclusion of evidence might occur if a judge ruled that certain testimony should be excluded because of the hearsay rule, but the ruling was erroneous because an exception to the rule applied.
Since a motion for a new trial usually goes to the same judge who presided over the initial trial, it's unusual for that judge to find a significant legal error. However, a judge who realizes that a particular ruling was in error might grant a new trial to avoid the case being overturned on appeal. That said, appellate courts tend to give trial court judges considerable leeway in their rulings. (For more information on appeals, see Appealing a Conviction.)
Courts may also grant new trial motions when certain kinds of new evidence have been discovered after conviction. But small, slightly helpful facts aren't enough. The new evidence generally must:
A judge or appellate court might grant a new trial based on newly discovered evidence if, for example, the defense had been looking for and finally locates the only witness who can corroborate the defendant's alibi.
A judge may also order a new trial if doing so could fix an injustice associated with the first trial. For example, in a 2013 federal case, a judge granted a motion for a new trial for two Mexican restaurant owners who had been convicted of harboring undocumented aliens for profit. After their conviction, the defense learned that one of the jurors made racist comments during the trial, calling the defendants "guilty wetbacks." In granting the new trial, the judge emphasized that defendants are entitled to 12 impartial jurors who decide cases based on the facts, not discriminatory beliefs. (United States v. Fuentes, No. 2:12-CR-50-DBH (D. Me 2013).)
Courts have also granted new trials to correct injustice when the defendant's constitutional rights (like the right to remain silent or confront witnesses) were violated, when the defendant didn't testify at trial due to fear of a codefendant, and when prosecutors have committed what is called a Brady violation by not turning over possibly exculpatory evidence to a defendant.
Defendants typically make motions for new trials after guilty verdicts. In some jurisdictions, the trial judge can order a new trial without a defendant asking. If the judge denies a motion for a new trial, the defendant can file an appeal asking a higher court to overrule the trial judge.
The prosecution's ability to move for a new trial depends on that jurisdiction's law and the result of the case. If trial resulted in an acquittal, the prosecution generally can't move for a new trial due to double jeopardy issues. However, in the case of a conviction, the prosecution might be able to ask for a new trial in the interests of fairness and justice. This situation occurred in the case of Maryland v. Adnan Syed (Sept. 19, 2022). After reinvestigating the 23-year-old murder case (made famous by the podcast "Serial"), the State's Attorney discovered numerous legal errors and violations made by its office, as well as the defense attorney, during the trial. The State's Attorney asked the court to vacate the conviction and order a new trial, stating that it had lost confidence in the integrity of the conviction.
If a judge grants a motion for a new trial, the case goes back almost to square one: The prosecution and defense can try the case again in front of a different jury.
For information on a similar post-trial remedy, see Can a judge acquit a defendant at a jury trial?