Whisk olive oil, lemon juice, garlic, thyme, and pepper together in a bowl and pour into a resealable plastic bag. Add chicken thighs, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for 30 minutes to 1 hour.
Preheat an outdoor grill for medium heat and lightly oil the grate.
Remove chicken thighs from the marinade and shake off excess. Discard the remaining marinade.
Cook until chicken is no longer pink in the center and the juices run clear, 4 to 7 minutes per side. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).