In a bowl, whisk together 1/4 cup low-sodium soy sauce, 1/4 cup water, 2 tablespoons brown sugar, 2 teaspoons rice vinegar, 1 teaspoon minced garlic, 1 teaspoon grated ginger, and 1 teaspoon sesame oil until the sugar is dissolved.
Pour half of the marinade over 4 salmon fillets (about 5 ounces each) in a shallow dish, reserving the other half of the marinade in the bowl. Turn the fillets to coat and let them marinate for 10 minutes.
Preheat the air fryer to 390°F.
In a small bowl, stir 2 teaspoons cornstarch into 1 tablespoon water until smooth to make a slurry.
Pour the reserved half of the marinade into a small saucepan and bring it to a simmer over medium heat. Stir in the cornstarch slurry and cook, stirring constantly, until the sauce thickens and becomes glossy (about 1–2 minutes). Remove from heat and set aside.
Remove the salmon from the marinade, letting excess drip back into the dish. Place the fillets in a single layer in the air fryer basket with space between them.
Air-fry the salmon at 390°F for 7 to 10 minutes, depending on thickness, until the fish is opaque through the center and flakes easily with a fork.
Transfer the cooked salmon to plates and drizzle or spoon the thickened teriyaki sauce over each fillet. Serve immediately.