The 25-year-old former England under-19 player has signed a three-year contract at The PROBIZ County Ground.
In his first stint wast Sussex in 2008 and 2009, he played seven first-class matches, 28 List A games and 25 Twenty20 games, scoring 1,204 runs and taking 39 wickets across all formats in the process.
In his final season, he helped Sussex to a one-day double in 2009 when the Sharks were victorious in both the Twenty20 Cup and Division One of the Pro40 competition, before joining Surrey for the start of the 2010 season.
He went on to captain Surrey to the Clydesdale Bank 40 title in 2011 as well as promotion back to Division One of the LV=County Championship, scoring over 1,000 first class runs in the process.
Hamilton-Brown said: “I am delighted to be returning to Sussex, a club where I was previously very happy. I would like to thank Mark Robinson, his coaching staff, the players, management and Board at Sussex for welcoming me back so warmly. There is nothing like feeling wanted to raise spirits and I fully intend to repay the confidence they have shown in me.
“I would like to thank Surrey for the opportunity they gave me to captain such an illustrious club at such a young age. At times it was very tough but I am immensely proud of what I achieved there, both as a captain and a player and specifically leading a special bunch of players to promotion and CB40 success.
“Losing my best friend Tom (Maynard) in June was indescribably painful. We were extremely close but ultimately, I know I will emerge from this awful tragedy with renewed strength and purpose.”
Sussex’s professional cricket manager Mark Robinson said: “Everyone at the club is delighted by Rory’s return. He has had a difficult few months and we believe we can play our part to assist and enable him to look to the future with renewed enthusiasm.
“He is a very talented cricketer who is yet to reach his full potential, which makes him such an exciting and dangerous player.”