Willock spent the second half of 2020-21 on loan at St James' Park and scored eight goals in 14 Premier League appearances for Steve Bruce's side.
The 21-year-old had netted only once in 40 Premier League games for the Gunners before his move to Newcastle, for whom he scored 16 minutes into his debut at home to Southampton.
Willock had an outstanding end to the campaign with goals in successive appearances against Tottenham, West Ham, Liverpool, Leicester City, Manchester City and Sheffield United to become the youngest player to net in six consecutive Premier League games.
In scoring on the final day of the season against Fulham, he matched Alan Shearer (in 1996) as the only Newcastle players to score in seven matches in a row in the competition – a record streak Willock could own outright with a strike on his second Magpies bow.
Bruce made no secret of his desire to keep Willock on a permanent basis but has had to bide his time as the former England youth international has featured sparingly for Arsenal in pre-season.
Willock becomes Newcastle's first signing of the transfer window, penning a "long-term" contract, which Bruce revealed to span six years. He did not join in time to face West Ham on Sunday.
WHY ARE ARSENAL SELLING WILLOCK?
Mikel Arteta's decision to cash in on Willock is a curious one given Arsenal lack the same sort of goal threat from midfield.
A scoring rate of one every 122.5 minutes for Newcastle ranked Willock third in the Premier League last season among players with multiple goals, behind Gareth Bale (83.9 mins) and Kelechi Iheanacho (121.3).
Meanwhile, no Arsenal midfielder netted more than Bukayo Saka's five goals in 32 matches. The Gunners last had a midfielder score eight in a season when Mesut Ozil matched that number in 33 2016-17 outings – before that, it was Aaron Ramsey (10 in 23) in 2013-14.
Even if Willock was not a guaranteed starter at Emirates Stadium, behind Emile Smith Rowe in the pecking order, his record as a substitute for Newcastle was hugely impressive.
The midfielder scored three goals in three appearances from the bench, averaging a goal every 15.7 minutes as a sub. Only 10 different Arsenal players have netted three or more times as a substitute across a single Premier League season.
However, Willock's exit appears to boost Arteta's hopes of bringing in a new signing in a similar position.
Leicester City's James Maddison and Martin Odegaard of Real Madrid – on loan at Arsenal last season – have both been linked and are far more effective creators than Willock.
Maddison last season created 2.2 chances per 90 minutes, including 0.3 'big chances', for an expected assists (xA) total of 0.15 per 90. His passing accuracy into the final third was 76.5 per cent.
Similarly, Odegaard, at Arsenal, created 2.1 chances, including 0.1 'big chances', for 0.20 xA per 90, completing 87.0 per cent of his passes into the final third.
For Newcastle, Willock created only 0.7 chances per 90 and no 'big chances', with an xA of 0.06 per 90, while his accuracy with passes into the final third was 68.8 per cent.