"We stay his execution pending the Supreme Court's resolution of Baze vs Rees," the court said, referring to the high court's decision last month to review whether lethal injections cause unacceptable pain.
Siebert's lawyers had argued that the drug combination used for lethal injection might interact with his medication for pancreatic cancer and hepatitis C and cause undue pain.
Siebert was convicted of the 1986 strangling deaths of Sherri Weathers, her two young sons and Weathers' friend Linda Jarman. Also convicted of murdering another woman, Siebert claims to have murdered others in various U.S. states.
Bryan Stevenson, director of the Equal Justice Initiative, which helped bring the suit on Siebert's behalf, said: "It would grossly inappropriate to carry out executions that may soon be declared unconstitutional by the Supreme Court."
Louella Kelley, Jarman's sister, lamented the ruling. "He's beaten the system again," she said in an interview. "He got himself educated in law while he's been in prison and his lawyers are very, very good. But all along he's been smarter than our justice system."
Alabama Gov. Bob Riley had said on Monday the execution would go ahead. It would have been the first since the beginning of a "creeping moratorium" that has halted executions in at least six U.S. states.