Abstract: A crib safety sack made of thin blanket fabric is provided. The sack is pocket-shaped to cover an end of a crib mattress. The top panel of the sack secures an infant in a back sleeping position on the surface of the crib mattress, which is the sleeping position recommended by experts for the prevention of SIDS. The sack acts as a blanket and also as a safety restraint keeping the infant securely in position through gentle force. The sack may be reversible, in that either a top or bottom panel of the sack may be positioned on top of the mattress. The sack preferably has at least one flap providing easy access.