Background: to compare several uterine biometric parameters at transvaginal ultrasound (TVUS) between adenomyosis and non-adenomyosis uteri and evaluate their role for the diagnosis of diffuse adenomyosis. Methods: prospective observational study conducted between the 1 February 2022 and the 30 April 2022. In this case, 56 patients with TVUS diagnosis of adenomyosis were included. A 1:1 ratio age and parity-matched group of non-adenomyosis patients was selected. We compared sonographic uterine biometric parameters (longitudinal (LD), anteroposterior (APD) and transverse (TD) diameters, volume, simple and complex diameter ratios) and investigated their diagnostic performance. Results: all sonographic parameters were significantly different between the study groups, except for TD/(LD+APD). Optimal cut-off values of APD and LD/APD showed the best sensitivity and specificity. APD diameter equal or superior to 39.5 mm (95% CI, 36.2–42.8) had sensitivity of 0.70 (95% CI, 0.57–0.80), specificity of 0.71 (95% CI, 0.59–0.82) and accuracy of 0.75 (95% CI, 0.66–0.84). LD/APD equal or inferior to 2.05 (95% CI, 1.96–2.13) showed sensitivity and specificity of 0.70 (95% CI, 0.57–0.80) each and accuracy of 0.72 (95% CI, 0.62–0.81). Conclusions: several biometric uterine parameters at TVUS in fertile-aged women were statistically different between adenomyosis and non-adenomyosis uteri, though their optimal cut-off values showed low accuracy in diagnosing adenomyosis.