Heat a wok or medium skillet over medium-high heat. Add the oil and swirl to coat the surface. When the oil is very hot but not smoking, add the shrimp and stir-fry until they begin to color on all sides, about 1 minute.
Add the pesto and stir-fry until it coats the shrimp and begins to cook, about 1 minute. Add the fish sauce and toss the shrimp for another 15 seconds to mix it in. Transfer the shrimp and sauce to a serving platter.