The results were scrapped because of several issues, Jecha Salim announced without saying when new elections will take place.
The US Embassy said in a statement that it is "gravely alarmed" and called for a reversal of the annulment.
"This action halted an orderly and peaceful election, as evaluated by observer missions from the US Embassy, European Union, Commonwealth, and Southern Africa Development Community, and a tabulation process nearing completion," the statement said. "We call for this announcement to be recalled, and urge all parties to maintain a commitment to a transparent and peaceful democratic process."
Tanzania was formed when Zanzibar and its northern island, Pemba, united with mainland Tanganyika in April 1964, three months after the islands' black majority overthrew the traditional Arab rulers. But for years, many have felt that Zanzibar, and its ancient capital, Stone Town, have been relegated to second-class status.
