Spurs head coach Ange Postecoglou made the Wales international his number one target following the departure of Harry Kane and secured the services of the young attacker late on transfer deadline day.
Johnson has signed a deal at Tottenham until the summer of 2029 after bringing his lengthy association at Forest to an end.
Versatile forward Johnson made his Forest debut in 2019, following in the footsteps of father David, and went on to make 108 appearances for his boyhood club, scoring 29 goals.
Johnson's arrival concludes a busy summer of incomings for Tottenham under Postecoglou, who has taken on a lead role on transfers in the absence of a director of football.
The Australian revealed on Friday that Spurs were close to finding a replacement for Fabio Paratici, who resigned from the role in April.
"Yeah, I think it will (be resolved soon). That's definitely the club's plan. It's fair to say that going into the window, there was no point in rushing that appointment, particularly with a new manager in.
"So far, the evidence, when you see the players we've brought in, I think we've navigated it pretty well."
Johnson's arrival was Tottenham's only incoming on transfer deadline day after proposed moves for Chelsea's Conor Gallagher and Lloyd Kelly of Bournemouth collapsed.
Spurs had to trim their squad before they could sanction any more incomings and even though defenders Japhet Tanganga and Sergio Reguilon left on loan to join Augsburg and Manchester United respectively, Eric Dier and Davinson Sanchez were unable to secure transfers away.
Pierre-Emile Hojbjerg also turned down an approach from Fulham and further talks with Atletico Madrid collapsed, while Tanguy Ndombele and Bryan Gil remained at Tottenham.
Manchester City and - of course - Chelsea were among those making moves on a typically frantic deadline day.
Chelsea signed 21-year-old winger Cole Palmer from City for STG40 million pounds ($A78m) taking the west London club's spending in this window to around ($A775m).
Central midfielders were also priorities for Liverpool, who brought Netherlands international Ryan Gravenberch from Bayern Munich in a deal worth 40 million euros ($A66m).
Liverpool sealed the deal for Gravenberch while rebuffing interest from Saudi Arabia in Egypt forward Mohamed Salah, who was reportedly the subject of a verbal offer totaling STG150 million pounds ($A292m) by Al-Ittihad.
Earlier on Friday, Liverpool manager Juergen Klopp said the 31-year-old Salah, who has 138 goals in 221 appearances in six seasons, was not for sale.
It was a busy day for United, who Reguilón on loan as cover for the injured Luke Shaw and Fenerbahce goalkeeper Altay Bayindir to back up Andre Onana. Centre back Jonny Evans also returned for a second stint at the club on a one-year deal.
Brighton, meanwhile, completed one of the most exciting deals of the window to bring in 20-year-old Spain winger Ansu Fati on a season-long loan from Barcelona.
Fati was once billed as Lionel Messi's successor at Barca, even taking his No.10 shirt, but struggled to make an impact since an injury in 2020 and hasn't been used as a regular starter under coach Xavi Hernandez.
Callum Hudson-Odoi left Chelsea after a 16-year association to join Forest, while Fulham signed Nigeria international midfielder Alex Iwobi from Everton for an undisclosed fee.
Forest also signed Belgian striker and ex-Liverpool star Divock Origi on a season-long loan from AC Milan.
In Europe, Barcelona signed Portuguese defender Joao Cancelo from Manchester City while they also took Joao Felix on loan from Atletico Madrid.
France forward Randal Kolo Muani joined Paris St Germain on a five-year deal from Eintracht Frankfurt while Manchester United striker Mason Greenwood moved to Spanish LaLiga side Getafe on loan until the end of the season.
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