Jeremiah St Phorose, aged 27, and Darrel Bryce, 31, left HMP Hatfield - an open prison - yesterday.
The prisoners are believed to have left together.
Both men are serving indefinite life sentences, with St Phorose jailed for offences including burglary and wounding, and Bryce jailed for robbery and possession of a firearm.
St Phorose, originally from Hull, has connections in Selby in North Yorkshire and in Humberside.
Bryce, originally from Nottinghamshire, has connections in the Nottingham and Worksop areas.
A South Yorkshire Police spokesman said: “A number of enquiries have already been conducted to locate the men, and officers would advise members of the public not to approach either man if seen, but to contact police immediately to enable their swift arrest and return to prison.”
Anyone with information should call South Yorkshire Police on 101 or Crimestoppers on 0800 555111.