The natural central midfielder is the first and only incoming of a quiet January transfer window for in the end for The Magpies.
Despite only being 21, Willock arrives in the North East with a wealth of Premier League experience, making 40 appearances for The Gunners over the last four seasons.
Willock has represented England at U16, U19, U20 and U21 levels, and will be competing directly alongside the likes Jonjo Shelvey, Isaac Hayden and Jeff Hendrick for a spot in head coach Steve Bruce's starting lineup.
With the deal only tied up in the dying minutes of the transfer window, Willock will not be available to make his Newcastle debut tomorrow night in a crunch home game against Crystal Palace, but it set to feature against Southampton on Saturday afternoon.
Willock: 'I can't wait to see everyone and be part of the Newcastle family'
After the move was finalised, Joe Willock said: “I'd like to thank all the supporters for their warm welcome and messages so far.
“This is a club with a massive history that I've grown up watching and I'll work my socks off for the team.
"I can't wait to see everyone and be part of the Newcastle family."
Bruce: 'He'll be a great addition to the squad'
Newcastle boss Steve Bruce added: "I'm delighted to bring Joe to Newcastle until the end of the season.
"He is a very exciting and talented young player who can get up and down the pitch and can score a goal from midfield.
"He has made a very decent impact at Arsenal after coming through the ranks and to make nearly 80 appearances for the club by the age of 21 says much about his development, ability and character.
"He is very keen to come here and test himself and he'll be a great addition to the squad."
How did this deal come about?
Steve Bruce reiterated in his pre-match press conference this morning that United had "one or two things bubbling along," with the priority being the arrival of a central midfielder, something which the club have been trying to pull off all month.
After an ambitious loan bid to sign Paris Saint Germain midfielder Idrissa Gueye was allegedly rejected by the player, and a move for Leicester's Hamza Choudhury failed to materialise, it meant Newcastle turned their full attention towards acquiring Willock's signature.
Willock agreed to join Newcastle this evening and made the journey up to the North East, before signing on the dotted line just before the transfer window slammed shut at 11pm.
One of the main reasons as to why the 21-year-old midfielder has decided to make the move to Tyneside is the opportunity to get regular first team football.
The Arsenal youngster has only made seven Premier League appearances for The Gunners this season, with just the one start.
When you combine the meteoric rise of Emile Smith Rowe, along with the arrival of Real Madrid's Martin Odegaard on loan, it means Willock's chances of becoming a regular in Mikel Arteta's side have been diminished even further.
What sort of player are Newcastle getting?
To get a further understanding on the sort of player Newcastle have on their hands, VAVEL spoke with Arsenal writer Jack Bibb to get his view on Willock's move to the North East.
"He’s got a great frame, he's strong and a willing runner at that.
"What I've noticed in particular with him is that he makes some great forward runs and has a great knack for getting a goal, which are probably his main bright spots.
On the 21-year-old's best position, Bibb said: "Willock's mainly featured for us in the Europa League, mostly playing as a number 10.
"However, he has also featured as a sitting midfielder alongside [Granit] Xhaka on the Premier League, so he can play in both.
"Personally, I'd say his best position is as an advanced midfielder where he can get forward, but doesn’t have all the creative burden."