Jeremiah St.Phorose, 27, and Darrel Bryce, 31, are believed to have left together after they were reported missing from the open prison in Thorne Road, Hatfield, yesterday morning.
Both men were serving indefinite life sentences for separate offences including burglary and wounding for St Phorose, and robbery and possession of a firearm for Bryce.
St.Phorose, originally from Hull, has connections in Selby, West Yorkshire and Humberside.
Bryce, originally from Nottinghamshire, has connections in the Nottingham and Worksop areas.
South Yorkshire Police is urging people not to approach either man but contact police immediately to enable their swift arrest and return to prison.
Anyone who believes they have seen or know the location of either man, should call 101 quoting incident number 80 of 28 March 2016, or call Crimestoppers anonymously on 0800 555111.