Spasticity causes stiffness in the masticatory muscles of individuals with cerebral palsy (CP), affecting the amplitude of mouth opening, making oral hygiene conditions difficult and predisposing these individuals to the risk of developing oral diseases. Objective: To evaluate the effect of a low-intensity photobiomodulation diode laser on the thick part of the masseter muscle in a child with spastic type CP. Method: The caregiver reported that the child had great difficulty in performing the oral hygiene, making avoidance movements of the head when the toothbrush touched the mucosa of the upper molar region in the mouth. The mother described the child's discomfort as extreme. The first ultrasound evaluation was performed at the first appointment, and the second evaluation after 6 sessions of photobiostimulation. The low-intensity Infrared Laser Diode, Ga-As-Al, was employed (λ = 808 ± 3 nm, 120 mW; Twin Flex Evolution Laser MMOptics São Paulo, Brazil), using 5.0 J/cm 2 dose energy/location, with 20 seconds exposure/site. The area of the masseter muscle was bilaterally irradiated towards the midpoint of its length and width. Six sessions were performed, with an interval of 7 days between them. Results: At the end of the sixth session of photobiostimulation, the mother reported that the child slept better, had reduction in involuntary movements performed by the jaw, and oral hygiene was possible now with no painful expression of the child. During palpation there was less stiffness in the bilateral masseter and an increased masseter thickness and increase in the amplitude of the mouth opening of 7 mm. Conclusion: The diode laser photobiostimulation appears to be effective in reducing spasticity in the masseter muscle of children with spastic type CP.