"I want to underscore the critical importance of the rollover of the Black Sea Grain Initiative on 18 March and of working to create the conditions to enable the greatest possible use of export infrastructures through the Black Sea," Guterres said during a visit to Kyiv.
The deal has made possible the export of 23 million tons of grain from three separate Ukrainian ports, Guterres said.
It has "contributed to lowering the global cost of food and has offered critical relief to people, who are also paying a high price for this war, particularly in the developing world," the UN chief said.
"Exports of Ukrainian -- as well as Russian -- food and fertilizers are essential to global food security and food prices," he added.
Russia's invasion last February saw Ukraine's Black Sea ports blocked by Moscow's warships until a deal last summer allowed for the safe passage of exports of critical grain supplies.
The deal, which will automatically renew on March 18 unless Moscow or Kyiv objects, was inked by Russia, Ukraine, Turkey and the United Nations last July.
Under last year's deal, Russia was promised the right to export fertilizer free from Western sanctions. Moscow insists that part of the agreement has not been respected.