BOTOX® (onabotulinumtoxinA) Treatment of Masseter Muscle Prominence

Inclusion Criteria * Participant has bilateral MMP (identical grades for left and right masseter), as determined at the Day 1 visit by the investigator using the MMPS * Participant has bilateral MMP, as determined at the Day 1 visit by the participant using the Masseter Muscle Prominence Scale-Participant (MMPS-P) * Body mass index (BMI) ≤ 30 kilogram/square meter (kg/m\^2) using the calculation: BMI = weight (kg) \[height (m\^2)\] * Female participants willing to minimize the risk of inducing pregnancy for the duration of the clinical study and follow-up period. A female participant is eligible to participate if she is not pregnant (has a negative urine pregnancy result prior to randomization), not breastfeeding, and at least one of the following conditions applies: 1. Not a woman of childbearing potential (WOCBP) OR 2. A WOCBP who agrees to follow the contraceptive guidance during the treatment and follow-up period * Able, as assessed by the investigator, and willing to follow study instructions and likely to complete all required study visits. Exclusion Criteria * Any medical condition that may put the participant at increased medical risk with exposure to BOTOX®, including diagnosed myasthenia gravis, Eaton-Lambert syndrome, amyotrophic lateral sclerosis, or any other condition that might interfere with neuromuscular function * Any uncontrolled medical condition * An anticipated need for surgery or overnight hospitalization during the study * An anticipated need for treatment with botulinum toxin of any serotype for any indication during the study (other than study intervention) * History of dental or surgical procedure for lower facial shaping or masseter muscle reduction * Prior mid-facial and/or lower facial treatment with nonpermanent soft tissue fillers, synthetic implantations, autologous fat transplantation, fat-reducing injectables, and/or skin-tightening laser treatments within 6 months of entry into the study * Current or planned dental or facial procedures during the study period (eg, braces, dental implants, and reconstructive or aesthetic surgery) that could interfere with MMPS, as determined by the investigator * Facial hair or scarring (eg, acne) significant enough to interfere with the 3D clinical photography assessment * Current enrollment in an investigational drug or device study or participation in such a study within 30 days of entry into this study * Prior exposure to botulinum toxin of any serotype to the masseter muscle or lower face at any time, or to any other part of the body within the 6 months prior to Day 1 * Current intraoral infection, including infection of the mouth or gums, or facial skin infection requiring medical treatment in the opinion of the investigator * History of or current Temporomandibular Joint Dysfunction (TMJD), or presence of signs/symptoms of possible TMJD, in the opinion of the investigator * Weakness of the masseter, pterygoid, or temporalis muscles due to trauma, facial nerve injury, or other condition that could interfere with normal chewing and jaw clenching, as determined by the investigator * Excess lower facial fat, loose or lax skin in lower face, or parotid gland prominence that could interfere with MMPS, as determined by the investigator * Significant asymmetry of left and right sides of the face that could prevent identical MMPS grading on both sides of the face, as determined by the investigator * Masseter prominence due to other etiologies (eg, parotid gland infection, parotiditis, malignancy) based upon findings from the oral examination.