Place the chicken thighs in a shallow dish or in a gallon-size ziploc bag.
Whisk together the pineapple juice, soy sauce, brown sugar, ginger and garlic until the brown sugar is dissolved (or mostly dissolved). Pour half of the marinade over the chicken in the bag so the chicken is evenly coated. Seal the bag and refrigerate for at least 4 hours or up to 24 hours.
Refrigerate the remaining marinade until the chicken is ready to be grilled.
Preheat grill to medium- or medium-high (I use 400 degrees on my pellet grill).
Bring the refrigerated reserved marinade to a simmer in a small saucepan on the stove. Simmer for 5-10 minutes (the longer it simmers the more reduced and slightly thickened it will be).
Place the chicken on the preheated grill (discard the marinade in the bag/dish) and cook until an instant-read thermometer registers 165 degrees F at the thickest part, about 7-8 minutes per side. During the last 3-4 minutes of cooking, baste the chicken liberally with the simmered sauce/marinade.
Serve the cooked chicken with the warm marinade (to be drizzled over - it's especially yummy if serving this chicken with cilantro lime rice or coconut rice).
Chicken: if you are like me, the thought of using chicken thighs may not be super exciting. I use boneless, skinless chicken breasts 99% of the time when chicken is called for in a recipe, but I've tried both chicken breasts and chicken thighs in this recipe, and chicken thighs are far, far, far superior. The flavor is deeper and the chicken is so incredibly juicy and tender. I buy boneless, skinless chicken thighs from Costco and trim as much fast as I can before using. Sauce: I've only made this recipe with low-sodium soy sauce. I have a feeling it would work quite well with tamari or coconut aminos (but I haven't tried it myself yet).