Nasir Ali was released on temporary licence from HMP Hatfield between 8.30am on October 18 and 3pm on October 20.
But he didn't attend his approved premises on the night of October 19 and has since failed to return to HMP Hatfield.
Ali, who has links across Sheffield, as well as Leeds and Manchester, was serving an indeterminate sentence after being convicted of conspiracy to murder and firearms offences in 2009.
Police have asked anyone who may have seen him to contact them on 999 immediately, but have warned against approaching him.
The 42-year-old is described as Asian and slim with a shaved head.
Anyone who thinks they may have seen him should contact South Yorkshire Police using incident number 909 of October 20.