The 31-year-old, who previously played for Spurs between 2007 and 2013, will not be immediately available for selection as the club confirmed in a statement.
"Gareth sustained a knee injury whilst playing for Wales on international duty earlier this month and this together with La Liga's later start compared to the Premier League means he has had a limited pre-season.
"We currently anticipate that he will be match fit after October's international break."
Tottenham's first game after the international break next month is away to Burnley on October 24 where Bale could don a Spurs jersey again.
Bale signed for Real Madrid for a then world record fee of $132.9 million and the Welshman has enjoyed a trophy-laden spell in the Spanish capital.
He has won four Champions League titles -- scoring the winning goal in two of those finals -- two La Liga titles and scored a stunning solo goal to win the 2014 Copa del Rey final against bitter rival Barcelona.
However, despite his success and 100 goals for Real, Bale has endured a fractured relationship with both the fans and manager Zinedine Zidane in recent seasons and will hope a move back to a club where he first made his name will revive his career.
"Our club wishes good luck to a player who has been part of one of the most successful stages of our history," Madrid said in a statement as the move was confirmed.
Bale's return to North London comes when Spurs are arguably in need of fresh blood, as Jose Mourinho begins his second season at the club.
The team are not blessed with a plethora of options up front, and Bale's injection of pace and keen eye for goal, should also result in an injection of enthusiasm from the club's supporters starved of success.
Spurs lost their opening game of the new EPL season at home to Everton last Sunday, though did have a welcome victory in Europa League qualifying on Thursday, coming from behind to defeat Bulgarian side Lokomotiv Plovdiv.
Bale has been joined from Real Madrid by the defender Sergio Reguilon. The 23-year-old Spanish international left back has signed a five-year deal with a buy back option for the Spanish giant.
After landing at Luton airport in a private jet on Friday, Bale was driven to the club's training ground, and was all smiles after being greeted by ecstatic supporters.
If Bale's performances on the field provide a similar amount of enjoyment, the feelgood factor may well last the entire season at Spurs, and perhaps continue into the next campaign.
He has pledged to bring his experience of winning trophies to Tottenham, without silverware since the English League Cup in 2008.
"I think by going to Madrid, winning trophies and going far with the national team, I feel like I have that winning mentality, how to win trophies," he told the Tottenham Hotspur website.
"Hopefully, I'll bring that to the dressing room, bring that belief that we can win a trophy, and the time to do that is this season, and not just one, to be fighting on every front possible. I want to bring that mentality back to Tottenham."
If he can match the form he showed for Spurs in his final season in 2012-13, that may well be a possibility.
Not only was the Welshman Tottenham's Player of the Year, but he was also awarded the PFA Players' Player of the Year, PFA Young Player of the Year, Football Writers' Association Footballer of the Year and Premier League Player of the Season accolades.
Tottenham's next game is away to Southampton on Sunday, which happens to be the team Bale left for Spurs back in 2007.