Hojbjerg becomes Jose Mourinho’s first summer signing, and he moves to North London after spending four seasons at the Saint Mary’s Stadium.
Hojbjerg had one year remaining on his Southampton contract and was stripped of the captaincy after making it clear to the Saints hierarchy that he was interested in pursuing a new challenge.
The 25-year-old Denmark international made 33 appearances as Ralph Hasenhuttl’s men finished 11th last season.
"What was very important for me was that I wanted to play at a club where I could see myself for many years, and Tottenham was just the one," Hojbjerg told Tottenham’s website.
"I'm very proud, very happy, excited. I think there is a fantastic future ahead for the club, and I wanted so much to be a part of that.
"Spurs has fantastic players, fantastic manager, a fantastic setup.
"I feel lucky and proud to be a part of it, and I want to give my everything, but I also want to learn a lot, to push for the next step, but still keeping that consistency."
Hojbjerg will inherit Jan Vertonghen’s number five jersey for the upcoming campaign.
In a separate transfer, right-back Kyle-Walker Peters, who spent the second half of last season on loan at Southampton, has made his move permanent from Spurs for £12m.
