Bees boss Andy Scott is pleased to land the 17-year-old and he is expected to go straight into the squad for this weekend's London derby with Millwall.
"I'm delighted to have signed John as he has been highly rated for several years as was proved by him signing for Tottenham at the age of 16," Scott told the club's official website.
"Spurs are looking for him to progress and force his way into the first-team reckoning and they feel that here at Brentford he will have the opportunity to show his worth. From our perspective it will give some added quality and dimension to our play."
Bostock is looking forward to his spell at the League One outfit.
"It's a good league and I'm looking forward to playing in it for Brentford"