Sexual identity is fluid. But just how fluid is it? How does such fluidity vary across demographic groups? How do mainstream measures fare in capturing the fluidity? Analyzing data from the United Kingdom (UK) Household Longitudinal Study (N = 22,673 individuals, each observed twice), this research note provides new population-wide evidence of sexual identity mobility—change and continuity in individuals’ sexual orientation identification—in the UK. Overall, 6.6% of the respondents changed their sexual identity reports between 2013 and 2019. Sexual identity mobility follows a convex pattern over the life course, with higher mobility rates at the two ends than in the middle of the age spectrum. Sexual identity mobility is more prevalent among women, ethnic minority individuals, and the less educated. Changes in people’s self-reported sexual identity are closely associated with changes in their partnership status and partner’s sex. However, inferring individuals’ sexual identity from their partner’s sex substantially underestimates the degree of sexual fluidity compared with people’s self-reported sexual identity. Our new evidence encourages researchers and data collectors to fully examine sexual identity mobility and consider its implications for measuring sexual identity.