Mix the seasonings in a bowl. Season the tri tip using the seasoning mix. Wrap it in plastic wrap and refrigerate for at least 2 hours, or overnight if possible.
Once the grill is preheated to at least 375° F, set the tri tip on the hot side of the grill and close the lid. Cook the roast for roughly 5 minutes per side to sear the meat all around and get some nice dark grill marks.
Once the searing is done, move the roast to the cooler side of the grill, off of the direct heat. If you have a leave-in grill thermometer probe, stick that into the thickest part of the meat and close the lid. Watch that the temperature in the grill doesn't go above 400° F.
The tri tip is done when the instant-read thermometer reads 130° F (54° C), about 15 minutes, for medium-rare. If you want medium, continue cooking to 135° F, about 20 minutes.
Take the meat off the grill and let it rest for at least 10 minutes wrapped in tin foil. This allows the meat to come up to the proper temperature.
To serve, slice the roast against the grain in thin slices.