Davis, who has been with the county since 2001, has retired from playing to replace Moores, after he left to become the National Academy director.
"I'm thrilled to be able to continue with Sussex in this new position and I'm really looking forward to the new challenges it will bring," said the Port Elizabeth-born player, who will be 34 next week.
"Coaching is something I'm very used to, and I feel it is a natural progression in my career. I'm very excited to be a part of the club in this new coaching era, and the 2006 season can't come soon enough!"
Sussex's professional cricket manager Mark Robinson said: "This is a fantastic opportunity for Mark and a great coup for the club.
"He has all the qualities to make an outstanding coach - strength, honesty, integrity and an ability to communicate at all levels.
"His experience both within the professional game as a player and most recently by running his own coaching academy in South Africa will make him a vital member of the coaching team."
Moores concluded his successful era as Sussex coach last month, guiding the county to third in this year's championship before taking up his new job as replacement for Rod Marsh as England's new national academy director.
It continues Sussex's successful policy of promoting from within. Moores was wicketkeeper-batsman for the county before becoming coach.