Despite increasing scientific and policy interest in sexual wellbeing, it remains poorly conceptualized. Many studies purporting to measure it instead measure related but distinct concepts, such as sexual satisfaction. This lack of conceptual clarity impedes understanding, measuring, and improving sexual wellbeing. We present qualitative research from multi-stage, mixed-methods work to develop a new measure of sexual wellbeing (Natsal-SW) for the fourth British National Survey of Sexual Attitudes & Lifestyles. Literature review and discussion generated a conceptual framework with seven proposed domains: respect, self-esteem, comfort, self-determination, safety and security, forgiveness, and resilience. Semi-structured interviews with 40 adults aged 18-64 then explored whether and how these domains aligned with participants' own understandings, experiences, and language of sexual wellbeing. Data were analyzed thematically. Participants conceptualized sexual wellbeing as distinct from sexual satisfaction and sexual health and as multidimensional, dynamic, and socially and structurally influenced. All seven proposed domains resonated with accounts of sexual wellbeing as a general construct. The personal salience of different domains and their dimensions varied between individuals (especially by gender and sexual orientation) and fluctuated individually over time. This study clarifies dimensions of domains that participants considered important, providing an empirical basis to inform development of a new measure of sexual wellbeing.