The 21-year-old had just returned from World Cup duty with the Young Lions this time last year when he joined Swindon Town on a season-long loan spell.
Pritchard made his Spurs debut on the final game of last season
And after impressing at the County Ground, he has agreed a similar spell at Griffin Park as well as extending his contract with Spurs until 2016.
"Alex is a player I have watched for many years," revealed Brentford boss Mark Warburton to BrentfordFC.co.uk.
"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."
Essex-born Pritchard has been with Spurs since he was 16 and made his first team debut for the club in the final game of last season after his spell at Swindon was over.
He had also previously featured in the Championship during a loan spell with Peterborough United in 2013.
