Boss Ronne Moore has been scouring the divisions for a player ever since Crewe recalled Adam Dugdale earlier this week due to an injury to Harry Davis.
Rovers are keen for more options in the centre of defence to provide alternatives to 40-year-old Ian Goodison and Ash Taylor and only last Saturday they brought in Huddersfield full-back Liam Ridehalgh on a three-month deal.
McNulty, 28, can play at either centre-back or at left-back, which is another position of concern for Tranmere given Danny Holmes' illness and Evan Horwood's groin injury.
Moore told Rovers' official website: "With Adam Dugdale returning to Crewe we are low on numbers in defence so I'm delighted to have signed Jim for a month.
"We've got a lot of young players on the pitch at the moment so Jim's experience will be useful.
"He's a player I've been keen on for a while. He can play in a couple of positions, is big and strong and uses the ball well."