....from Sky Bet League Two side Luton Town on a contract which will expire at the end of the 2016/17 season.
The commanding central defender has been on loan at Prenton Park since the middle of October, making a dozen appearances for the club.
During his loan spell, McNulty has picked up a number of man of the match awards, including in Bank Holiday Monday’s win over Altrincham, and scored his first goal for the club to set up a 2-1 victory away at Macclesfield Town on Boxing Day.
Manager Gary Brabin is delighted to have made the defender’s move permanent, saying: "I’m really pleased that we’ve agreed a permanent deal with Steve to remain at the club.
"He’s a leader, has experience of promotions out of this league and has done really well in the couple of months that he has been here. He has settled in nicely with the rest of the squad and I’m delighted to extend his stay at the club."