In a mixing bowl, whisk together 1/2 cup teriyaki sauce, 1/4 cup pineapple juice, 3 chopped green onions, 3 pressed garlic cloves, and 2 tablespoons vegetable oil until combined.
Poke the 2 lb chicken breasts all over with a fork to help the marinade penetrate.
Place the chicken in a zip-top bag or shallow dish and pour the marinade over it, turning to coat completely. Refrigerate for at least 30 minutes and up to 4 hours.
Preheat the grill to medium and lightly oil the grates to prevent sticking.
Grill the chicken 4–6 minutes per side, depending on thickness, until an instant-read thermometer registers 165°F (74°C).
Remove the chicken from the grill and let it rest 5 minutes before slicing and serving.