Greitens issued the stay of execution for Marcellus Williams, 48, just over four hours before he was scheduled to die by lethal injection in a Bonne Terre state prison for the stabbing death of former St. Louis Post-Dispatch reporter Felicia Gayle during an August 1998 robbery at her home.
Greitens said in a statement he would appoint a Board of Inquiry to examine the new DNA evidence and recommend whether he should commute Williams' death sentence.
The stay came as opposition to the execution mounted. More than 200,000 people had signed an online petition asking Greitens to intervene, and a rally protesting the execution was planned in St. Louis later on Tuesday, featuring speakers such as the head of the Missouri NAACP.
"The DNA evidence says he's innocent," Kent Gipson, Williams' attorney, said earlier on Tuesday.