[Purpose] It is unknown whether handgrip strength dominance is related to the size of the forearm flexor muscles. The purpose of the present study was to investigate the relationship between side-by-side differences in handgrip strength and forearm muscle thickness. [Subjects] Thirty-one young women (26 right handed and 5 left handed) between the ages of 20 and 33 years volunteered to participate. [Methods] Two muscle thicknesses (forearm-ulna and forearm-radius muscle thicknesses) were measured using B-mode ultrasound at the anterior forearm on both sides of the body. Handgrip strength was also measured on both sides. [Results] The side-by-side difference in handgrip strength was 10.2% for the right-handed group, meaning the right hand was stronger. However, the left hand of the left-handed group was 7.8% stronger compared with their right hand. There was a significant positive correlation between side-by-side differences in handgrip strength and forearm-ulna muscle thickness (r = 0.765) and between handgrip strength and forearm-radius muscle thickness (r = 0.622). [Conclusion] Our results indicate that side-by-side differences in forearm muscle size may strongly contribute to handgrip strength dominance.