Thirty-seven-year-old Terrell Johnson has been serving a life sentence in the July 1994 murder of Verna Robinson of Hazelwood.
The jury in the retrial delivered a not guilty verdict late Wednesday afternoon, prompting Johnson to break out in tears.
Authorities in Allegheny County said Johnson killed Robinson to prevent her from testifying against a friend in a drive-by shooting several months earlier.
Johnson was granted a new trial when his attorneys produced a witness who said a woman who identified him as the gunman wasn't at the scene.
It's not clear when Johnson will be released from prison.