Moore came through the ranks at Southampton before joining Farnborough Town and then, in 2009, Brentford.
He played in 52 matches for the Bees last season, keeping 13 clean sheets in the process, and his progress was followed closely by Cardiff boss Malky Mackay.
Mackay told the City website: "We have been watching Simon for a year or so - he is a real up-and-coming goalkeeper.
"My thanks go to Brentford for the way they went about their business and we are delighted to get Simon in for the start of our season.
"He is one for the future and we have three quality senior goalkeepers in our squad."
Moore, 23, said: "I'm delighted to be at Cardiff City - to play in the Premier League has always been my goal.
"It's exciting to think that I'll be working with quality goalkeepers like David Marshall and Joe Lewis on a daily basis and I know that I can develop and learn from them."