Ms Nyembezi-Heita has spent almost six years at the helm, which included a bruising dispute with Kumba Iron Ore over the pricing of supplies.
She will also step down as an executive director, effective 18 February, 2014.
No reason was given for her resignation.
"Although challenging, over the last six years as a management team, we have achieved significant advances in a number of areas," Ms Nyembezi-Heita said in a statement. "Our safety performance is now world-class and we have also taken huge strides towards meeting our environmental obligations. There is steady progress in our operational improvement programmes with pleasing results in places. As the market turns, we can expect a marked improvement in our financial performance, aided in no small measure by the recent settlement with Kumba."
Chairman Mpho Makwana thanked her for her tremendous contribution to the company during an "exceptionally challenging period" not only for ArcelorMittal South Africa, but the steel industry generally.
He wished her success in the next chapter of her career and commented that the firm is in a considerably better position than when Ms Nyembezi-Heita joined the group.
"On behalf of the Board of ArcelorMittal South Africa and the ArcelorMittal Group, we express our thanks for her unstinting dedication, guidance as well as her calm and determined leadership," he said.
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