Tennessee's prison chief is stepping down just days after the failed execution of Christa Pike left her on a ventilator.

Tennessee Department of Correction Commissioner Frank Strada said that his resignation "is in the best interests" of the state ... as it conducts a third-party review of why the execution was botched.

Tennessee Governor Bill Lee -- who announced the resignation Saturday -- also spoke out about the decision for Strada to step aside, saying he was "grateful for his commitment" to ensure a smooth transition.

Lee said ... "I appreciate his willingness to put the interests of Tennesseans first during this difficult moment."

Strada's resignation comes after 2 lethal injections during Pike's execution left her snoring and unconscious. Tori Gessner -- an anchor for WKRN news in Tennessee -- joined us on "TMZ Live" Thursday, detailing the disturbing situation as she saw it.

Pike's legal team has since shared that their client is on a ventilator and remains unconscious.