The 20-year-old, who signed a three-year deal with the Foxes in September, had been linked with a move to Manchester United in the summer.
Brentford sporting director Mark Warburton told the club's official website: "Liam is a player I have tracked and monitored for the last two to three years. He has come through the same youth system that produced Jeffrey Schlupp, who I am sure Brentford fans will remember.
"Liam has had an excellent academy education and he has developed in to a fine young defensive player. He is comfortable both in central defence and at right-back and also, crucially, in defensive midfield.
"He has already achieved a lot this season, playing in the Championship with Leicester, and other clubs were looking at him. We are delighted he chose Brentford."
Moore has made 14 first-team appearances for the Foxes this season.