The South African finished at 12-under-par 276 and became the first golfer to win back-to-back SAA Opens since Gary Player, another native son, in 1976 and 1977.
It was also the first time that a player successfully defended a title on the European Tour since Ernie Els, did so at the Heineken Classic in Australia in 2002 and 2003.
"Those guys are legends in South African sports and it is fantastic for me to have done something that each of them have done," Immelman said.
Scotland's Alastair Forsyth and England's Steve Webster shared second at 279 after rounds of 68 and 71, respectively.
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Michael Sam cut from the Dallas Cowboys