Heat gas or charcoal grill. In large bowl, mix turkey, ranch dressing and taco seasoning mix. Shape mixture into 4 patties, about 4 inches wide and 3/4 inch thick.
Place patties on grill over medium heat. Cover grill; cook 13 to 15 minutes, carefully turning once, until meat thermometer inserted in center of patties reads at least 165°F. Top each burger with 1 slice cheese and place buns, cut sides down, on grill; cover grill and cook 1 to 2 minutes or until cheese is melted and buns are lightly toasted.
Place burgers on bun bottoms. Cover with bun tops.