De Bruyn, 29, made his Test debut last year and has since hit three hundreds and three half-centuries in the South African domestic season.
In the field, he is also a fast-medium bowler and Tom Moody, Worcestershire's director of cricket, said de Bruyn was just the kinds of player the midlands county were after.
"We have spent some time looking for a batting all-rounder to fill one of our overseas spots, and I believe we have found the right person in Zander," said Moody, whose team were relegated from Division One of the County Championship last summer following promotion the previous year.
"He has had an outstanding first-class season in South Africa and is determined to make the grade at international level," Moody, himself a former Australia batting all-rounder added.
"He will bring the right qualities to our squad, and I have no doubt he will make a positive impact throughout the season."
De Bruyn, who was dropped from the South Africa team after featuring in the first Test of the recent 2-1 home series defeat against England, was upbeat about his prospects at New Road.
"I am really looking forward to the opportunity of playing with Worcestershire and to contributing to a successful season," he said.
"Clearly the county has a strong and competitive squad and is ambitious to build on the progress made over the last few years. The opportunity could not have come at a better time in my career, and I intend to make it count for the Worcestershire supporters."
However, De Bruyn's signing is still subject to official clearance from the United Cricket Board of South Africa.
Worcestershire hope to have Shoaib Akhtar on their books from July 1 but have yet to announce who will stand-in for the Pakistan fast bowler as their second overseas player when the season gets underway in April.