The Magpies have agreed a fee and personal terms for the central defender, who is viewed as one of the most talented young prospect in Scotland.
McArthur was handed his senior debut by former United goalkeeper Tommy Wright in October last year aged just 16.
Charlie McArthur tipped for ‘big, big future’
And speaking after that game, Wright said: “The biggest compliment I can give Charlie is that the lads treat him like a 26-year-old.
“Since I’ve been in here, he’s trained all pre-season with us. We looked at him last season and he’s a dream to work with.
“I don’t want to stick labels on how good he is but he’s very, very good and will have a great career.
“He can be proud of his debut. He has a big, big future ahead of him.”
McArthur made four appearances in total for Killie last season before captaining Scotland’s under-17s at the European Championships in Israel.
The defender has been described as “very level headed” and a “good footballing centre-back”.
Over the last 12 months, Liverpool, Manchester City, Brighton, Wolves and West Ham have been credited with an interest.
Building for the future
McArthur is set to become Newcastle’s fourth summer signing behind Matt Targett, Nick Pope and Alex Murphy.
United announced last month that Murphy, who turns 18 on Saturday, will link up with Elliott Dickman’s under-23s after signing from Galway United.
Like McArthur, signing Murphy, who has captained Republic of Ireland under-18s, is regarded as a big coup.
Academy manager Steve Harper said: “We are very pleased to have secured a deal to bring Alex to Newcastle United.
“He is a very promising young player who has attracted a lot of attention from clubs. It is a real coup for our academy and is testament to the direction the club is going in under our owners.
“I would like to thank everyone involved with Galway United for their assistance. The club has been a pleasure to deal with and I wish John Caulfield and his players the very best of luck for the rest of the season.”