Lay pork tenderloins on a cutting board and remove silver skins with a sharp knife. Place pork in a large resealable plastic bag.

Combine soy sauce, olive oil, brown sugar, and rosemary in a small bowl. Pour marinade over pork in the bag, squeeze out excess air, and seal the bag. Marinate in the refrigerator for 3 to 4 hours.

Remove pork from the marinade and shake off excess. Discard the remaining marinade. Place pork on a plate and allow to come to room temperature, about 1 hour.

Preheat an outdoor grill for medium-high heat and lightly oil the grate.

Place pork on the preheated grill and grill until slightly pink in the center, 20 to 30 minutes. An instant-read thermometer inserted into the center should read at least 145 degrees F (63 degrees C) for medium rare, or 160 degrees F (71 degrees C) for medium. Allow pork to rest 5 minutes before slicing into medallions for serving.