ISTANBUL—Standard & Poor’s revised Turkey’s credit-rating outlook to stable from negative Friday, citing the resilience of the Turkish economy in the face of global and local threats to the country’s stability.

The New York-based ratings firm kept steady Turkey’s sovereign rating at BB+, one notch below investment grade, saying the economy still faced challenges ranging from political turbulence to a resurgent conflict with Kurdish separatists and geopolitical risk from the war in neighboring Syria.

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