The 37-year-old had been on a season-long loan with Forest's fellow Championship side Watford.
But Murray has only made one league start for the Hornets and has now opted to swap the top end of the second tier for the chance to link up again with his old Brighton boss Chris Hughton.
"Glenn is a player I know very well," said Hughton, now in charge of Forest.
"He has an excellent pedigree at this level. He will add leadership and experience to our squad between now and the end of the season.
"He is a proven goalscorer at Premier League and Championship level and has continued to keep himself fit."
Forest are 21st in the Championship table after Saturday's goalless draw with Barnsley, just one place and three points clear of safety as they bid to avoid relegation to League One.