Abstract Introduction The potential consequences of being assertive or non-assertive for people’s sexual health and sexual well-being highlight the importance of assessing sexual assertiveness (SA). The currently available measures have limitations because they do not take recent social changes into account, they are designed to only assess women, and/or they ignore several components of SA. This study tests the psychometric properties of the Assertiveness in Sexual Relations Questionnaire (ASRQ). Methods Thus, 2370 participants (aged 18–69 years) of Spain completed the ASRQ, along with other scales that assesses related dimensions (e.g., family values in relation to sexual assertiveness, sexual esteem). Data were collected during 2020. Results Exploratory factor analysis yielded a four-factor structure: Assertive behavior as initiative, sub-assertive behavior, overly assertive behavior as initiative, and assertive behavior as a response, which was verified by confirmatory factor analysis (CFA). A multigroup CFA was also conducted in men and women, confirming the adequacy of this four-factor structure across genders. The reliability of the factors ranged from 0.72 to 0.87. Similarly, correlations with related scales were mostly significant and in the expected direction. Conclusions The psychometric results obtained support the use of the ASRQ as a reliable and valid measure to assess sexual assertiveness in both men and women. Policy Implications The availability of an instrument to assess sexual assertiveness, whose psychometric properties have been satisfactorily tested, benefits society by contributing to the improvement of the sexual health of the population, allowing for more effective interventions and the early detection of skills that contribute to the establishment of risky sexual interactions.