Marinate the boneless chicken thighs in tamari, lemon juice, salt, and garlic powder for 30 minutes to 4 hours in the fridge. Preheat the grill to medium-high heat and lightly oil the grates.
Grill the chicken over direct heat until nicely charred, about 4 to 6 minutes per side. Move the chicken to indirect heat and cook until the internal temperature reaches at least 165°F. Let the chicken rest for 5 minutes before slicing and serving.