Make Tzatziki Sauce: Combine all tzatziki ingredients in a small bowl then refrigerate.
Mix chicken marinade ingredients and pour over chicken. Marinate for at least 1-4 hours.
Grill Chicken: Heat grill to medium heat, then lift chicken from marinade, letting it drip for a few seconds before placing on hot grill. Cook on both sides for a few minutes, flipping once, until cooked through. Rest on a plate then chop into bite-size pieces.
Assemble: Toast or warm your pitas then add chicken, lettuce, tomato, red onion, cucumber, and drizzle with homemade tzatziki sauce.
Notes
For best results, allow the chicken to marinate for the full 4 hours to maximize flavor and tenderness.