Sprinkle both sides of tilapia fillets with 5-spice powder. Combine soy sauce and brown sugar in a small bowl.
Heat oil in a large non-stick skillet over medium-high heat. Add the tilapia and cook until the outer edges are opaque, about 2 minutes. Reduce heat to medium, turn the fish over, stir the soy mixture and pour into the pan.
Bring the sauce to a boil and cook until the fish is cooked through and the sauce has thickened slightly, about 2 minutes more.
Add scallions and remove from the heat. Serve the fish drizzled with the sauce.