The 20-year old midfielder spent the first half of the campaign at Meadow Lane and impressed so much that League One County wanted him back for the remainder of the season.
The signing of Scotland U21 international McGregor, who has scored 12 goals in 27 appearances for the Magpies, delighted chief executive Jim Rodwell.
"We are obviously delighted to have secured Callum's signature for the rest of the season and I am sure that our fans will share in the excitement and anticipation of seeing him back in action for us," he said.
"I have to say a big thank you to Celtic for the way that they have worked with us throughout not only the initial negotiations to bring Callum here in the first place, but also since his loan deal ended and in light of interest from a number of high-profile clubs.
"It is understandable that Callum has been attracting the interest of other clubs because he is obviously a very talented player with a bright future ahead of him, but Celtic kept us informed at every stage of the process and were fair and honest throughout.
"Now we are just looking forward to seeing Callum pull on the black and white shirt once again and continue his development with us here at Meadow Lane over the coming weeks and months."
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