The 21-year-gasket left arm has represented South Africa in the three formats of the game and played for Kent in England in 2009.
Parnell, who is South African team for the next World Cup, said: "I am very excited to play for Sussex."
"I think this will be a great learning curve and I will certainly gain more experience by playing with experienced professionals as well as international cricketers."
Sussex professional cricket manager Mark Robinson said: "We are delighted that one of the talents of South Africa has achieved."
"It gives us not only a threat of taking wicket, but the potential for the exploitation of precious stones."
