Rompilla v. Beard, 545 U.S. 374 (2005), is a case decided by the Supreme Court of the United States on June 20, 2005.

In a majority opinion authored by Justice David Souter, the Court held 5–4 that the petitioner, convicted murderer Ronald Rompilla, had received ineffective assistance of counsel due to his lawyer's failure to adequately investigate and obtain evidence that the lawyer knew the prosecution would likely use against the defendant.

It therefore reversed the prior ruling to the contrary by the United States Court of Appeals for the Third Circuit, and overturned Rompilla's death sentence as a result.

It was the third major case in five years in which the Supreme Court had invalidated a death sentence on the grounds that the defendant had received ineffective assistance of counsel.

O'Connor joined the majority in this case, which attracted attention in part because the author of the Third Circuit decision that the Supreme Court overturned in this case was Judge Samuel Alito, who replaced O'Connor on the Supreme Court the following year.