Context Neck pain (NP), neck injuries, and concussions are more prevalent in female athletes than in their male counterparts. Females exhibit less neck girth, strength, and stiffness against a perturbation. As part of the clinical examination for individuals with NP, ultrasound (US)–based imaging of the cervical muscles has become common. Muscle size or thickness and stiffness can be measured with US-based B-mode and shear-wave elastography (SWE), respectively. Information on reliability, normative values, and sex differences based on US-based muscle size or thickness and stiffness in young and athletic individuals is limited. Objective To evaluate sex differences in US-based muscle size or thickness and biomechanical properties of the cervical-flexor and -extensor muscles. Design Cross-sectional study. Setting Laboratory. Patients or Other Participants A total of 13 women (age = 23.7 ± 1.9 years, height = 167.1 ± 6.1 cm, mass = 63.8 ± 5.6 kg) and 11 men (age = 25.6 ± 4.9 years, height = 178.7 ± 8.3 cm, mass = 78.9 ± 12.0 kg). Main Outcome Measure(s) The same examiner collected all measures, using US B-mode to scan the cross-sectional area and thickness of the longus colli (LC), sternocleidomastoid (SCM), cervical-extensor muscles, and upper trapezius (UT) muscle. The US SWE-mode was used to measure the stiffness of the SCM and UT. Independent t tests or Mann-Whitney U tests were calculated to determine sex differences. The intraclass correlation coefficient (ICC) measured intrarater test-retest reliability. Results Men had thicker SCMs than women (P = .01). No sex differences were present for longus colli cross-sectional area, cervical-extensor muscle thickness, or UT thickness (P > .05). In addition, no sex differences were evident for SCM (P = .302) or UT (P = .703) SWE stiffness. Reliability was good to excellent (ICC = 0.715–0.890) except for SCM SWE stiffness (ICC = 0.554). Conclusions The only sex difference was in SCM thickness. However, smaller SCMs in women did not result in less SCM SWE stiffnes...