The Madrid-based UNWTO will provide expert advice to help the Turkish government with its communications strategy and marketing, a spokeswoman said.
The agency will also assist Turkish efforts to boost the number of direct flights from key European markets, she added.
"Turkey is absolutely important for us," the UNWTO's Jordanian secretary general Taleb Rifai said after he and Turkish Tourism Minister Nabi Avci signed the agreement in Madrid.
"Together we are sending a message to the whole world. Turkey is back in business. Turkey is ready to receive its visitors."
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Major cities in Turkey descended into battlegrounds on the night of July 15 during a military coup attempt against President Recep Tayyip Erdogan.
The uprising has further undermined Turkey's tourism industry, already hit by repeated suicide bombings over the past year that have spooked visitors, including one in June that killed 47 people at Istanbul's main airport.
Avci tried to play down recent unrest, saying that jihadist attacks were a global problem that does not just affect Turkey.
He told reporters that the failed coup -- and the subsequent crackdown that has seen tens of thousands of people detained and sacked -- had "nothing to do" with tourism.
"Everyday life continues completely normally, all airports are open," he said.
"Commercial activities continue without any problems," he added.
"This state of emergency will only affect those implicated in the coup attempt.