Officials said the Razoni cargo ship, registered in Sierra Leone, was making its way through a specially cleared corridor in the mine-infested waters of the Black Sea with 26,000 tonnes of maize on board.
"It is expected in Istanbul on August 2. It will then continue its journey after it has been inspected in Istanbul," the Turkish foreign minister said in a statement.
Other convoys would follow, respecting the maritime corridor and the agreed formalities, the statement said.
United Nations chief Antonio Guterres, who brokered the plan along with Turkey, welcomed the announcement while Kyiv said it would bring "relief for the world" if Moscow held up its side of the accord.
The five-month halt of deliveries from war-torn Ukraine -- one of the world's biggest grain exporters -- has contributed to soaring food prices, hitting the world's poorest nations especially hard.
Last month, Ukraine and Russia signed the breakthrough pact -- the first signficant accord involving the warring sides since the invasion -- with Turkey and the United Nations aimed at getting millions of tonnes of trapped Ukrainian grain to world markets.
But Russian strikes on the Odessa port the day after the deal was signed sparked outrage from Ukraine's allies and cast doubt over the accord.
Guterres, according to a UN statement, "hopes that this will be the first of many commercial ships moving in accordance with the initiative signed, and that this will bring much-needed stability and relief to global food security, especially in the most fragile humanitarian contexts".