The 21-year-old has signed a contract until 2016 and will be hoping to gain more first-team experience during his temporary stay with the Championship club.
Pritchard made his Spurs debut against Aston Villa on the final day of last season, but previously was loaned out to Peterborough United and Swindon Town.
He was shortlisted for the Sky Bet League 1 Player of the Year award after scoring six goals in 44 appearances for Swindon last season.
Brentford boss Mark Warburton has welcomed the arrival of Pritchard, who has an option to terminate the deal during the January transfer window.
"Alex is a player I have watched for many years," Warburton told the club's official website. "I have followed his career at Tottenham from a young age.
"He is very good tactically and technically with excellent dead ball delivery. He showed his quality while he was on loan at Swindon last year.
"Alex will add to the quality in our squad, which is taking shape nicely ahead of the start of the season.
"This deal is testament to the work of sporting director Frank McParland and his team over the summer. We fought off competition from a number of Championship clubs to get Alex to Griffin Park."