Spurs, who have yet to disclose the amount paid or the length of contract, fought off competition from Sevilla for the 20-year-old midfielder.
Regarded as one of the most talented youngsters in Germany's Bundesliga, Boateng made 42 appearances for Hertha and has been capped by Germany at youth and Under-21 level as well as being voted the country's best youngster in July last year.
A fee has yet to be confirmed but the two clubs were believed to have agreed a deal in the region of £5.4million last week.
"For me it is amazing to be in England," Boateng told Spurs' official website.
"To be in London and playing for Spurs will be great for me. It has been my ambition to play in England, I am 20 now and am about to quickly realise my dream.
"I know that Spurs are a team who like to play offensive football. The club want to climb higher next season and I want to be a part of it.
"I watched the Premier League regularly in Germany, it is the most famous league in the world and I have always enjoyed it."
Having already strengthened his squad by acquiring Darren Bent, Gareth Bale, Adel Taarabt and Younes Kaboul this summer, Spurs boss Martin Jol is confident he has added another talented youngster.
The Dutchman explained: "Kevin is a talented player who plays for the under-21 side of his country.
"You could compare him with the likes of Aaron Lennon and Tom Huddlestone in that he is an exciting player for now and the future.
"He is an attacking midfield player, very strong and that is one of his main assets.
"In Germany he is regarded as one of the biggest talents, we recognised this and went in for him."
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