Mix onion powder, garlic powder, smoked paprika, chili powder, brown sugar, rosemary, pepper, soy sauce, and olive oil in a bowl.
Place the tri-tip in an aluminum foil pan or resealable plastic bag and pour the marinade over it. Seal the bag and massage or rub the marinade into the meat. Tightly seal it with plastic wrap and marinate it in the fridge for around an hour (or up to 24 hours for added flavor).
Remove the tri-tip from the marinade and let it come to room temperature for 20-30 minutes before grilling.
Set your grill to 225°F (107°C) to slow cook for a tender, juicy, smoky tri-tip. Place the tri-tip directly on the grill’s grates and smoke for 1-2 hours or until the internal temperature reaches about 110-115°F (43-46℃).At this stage the temperature should be around 130-135°F (54-57°C). For medium, aim for 140-145°F (60-63°C).
Remove the tri-tip and increase the temperature to medium-high (450°F-500°F/230-260℃). Sear your tri-tip for 2-3 minutes on each side or until it develops good grill marks, and sear on the outside.
Remove from the grill, cover with foil so it stays warm, and allow it to rest for 10-15 minutes.
Cut against the grain (the direction of the muscle fibers) into thin, pencil-sized slices for deliciously tender meat.