In a small mixing bowl, whisk together soy sauce, olive oil, brown sugar, lemon juice, Dijon mustard, garlic powder, and black pepper.
Place salmon fillets in a resealable plastic bag or shallow dish.
Pour the marinade over the salmon, making sure all pieces are coated.
Seal or cover and refrigerate for 30 minutes to 1 hour.
Preheat grill to medium-high heat.
Grill salmon for 4-5 minutes per side, or until cooked through and easily flakes with a fork.
Serve immediately with your favorite side dishes.