Eskom said in a statement that the decision was taken following a special meeting between the company's board and De Ruyter.
"The board... resolved that... he will be released from his position with immediate effect," it said.
De Ruyter had already resigned from the struggling utility in December, but had agreed to stay on the job till 31 March as the utility tried to find a replacement.
Earlier on Wednesday, government announced plans to take on more than half of Eskom's R423bn debt over the next three years to help strengthen its balance sheet and operations and enable it to restructure.
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