The PFA Players' Player and Young Player of the Year has been constantly linked with a move away from White Hart Lane after a phenomenal season, with Real Madrid reportedly leading the pursuit.
But Villas-Boas says he has been given assurances by chairman Daniel Levy that the 23-year-old will not be sold.
"The chairman said that we are not open to offers, whatever they are, and that the player will stay," he told Portuguese newspaper O Jogo.
"In football anything can happen, but these are the guarantees given to me.
"There was a moment last season when Gareth came to me because he was not comfortable out on the pitch.
"I do not know if it was my response or that he was glad to relieve the stress, but after that he became confident and played with freedom.
"He felt important and finished the year in style."
Meanwhile, Jose Mourinho's return to Chelsea means that Villas-Boas will go head-to-head with his old boss next season when he faces the Europa League winners.
The 35-year-old served as an assistant to Mourinho at Porto, Chelsea and Inter Milan and eventually occupied the Stamford Bridge hotseat himself during an unsuccessful eight-month spell.
And while he admits that he and the two-time Champions League winner have had little communication of late, he still has the utmost respect for Mourinho.
He said: "Although we've grown apart, I think we are unified by professional respect."