The 21-year-old, who has signed a long-term deal, enjoyed a stellar loan spell at St James’ Park last season, scoring eight goals in just 14 league games.
That goalscoring run saw him become the youngest ever player to score in seven successive Premier League matches, helping the Magpies finish 12th, and Willock revealed his delight at joining. He said: ‘I’m buzzing to be here. I loved my time here last season and I’m really happy to be back and to be a Newcastle United player.
‘Leaving Arsenal is obviously a big step, and I’d like to thank everyone involved with the club for all their support over the years, but this is the right move to the right club at the right time for me.
‘I only got one chance to play in front of a smaller number of Newcastle fans as a home player last season, so I can’t wait to step out in front of a packed St. James’ Park and experience that atmosphere. Come on, you Maggies!’
The club had hoped Willock would be available for the season opener against West Ham on Sunday, if they had registered him by midday on Friday, but Newcastle have now confirmed he will be available for next week’s match against Aston Villa.