Objectives: To assess whether educational differentials in three key physical activity (PA) domains vary by age, sex and ethnicity., Design: National cross-sectional survey., Setting: UK., Participants: Altogether 40 270 participants, aged 20 years and over, from the UK Household Longitudinal Study with information on education, PA and demographics collected in 2013-2015., Outcome Measures: Participation in active travel (AT), occupational activity (OA) and leisure time physical activity (LTPA) at the time of assessment., Results: Lower educational attainment was associated with higher AT and OA, but lower weekly LTPA activity; these associations were modified by sex, ethnicity and age. Education-related differences in AT were larger for women-the difference in predicted probability of activity between the highest and the lowest education groups was -10% in women (95% CI: -11.9% to 7.9%) and -3% in men (-4.8% to -0.4%). Education-related differences in OA were larger among men -35% (-36.9% to -32.4%) than women -17% (-19.4% to -15.0%). Finally, education-related differences in moderate-to-vigorous LTPA varied by ethnicity; for example, differences were 17% (16.2% to 18.7%) for white individuals compared with 6% (0.6% to 11.6%) for black individuals., Conclusions: Educational differences in PA vary by domain and are modified by age, sex and ethnicity. A better understanding of physically inactive subgroups may aid development of interventions to both increase activity levels and reduce health inequalities., Competing Interests: Competing interests: None declared., (© Author(s) (or their employer(s)) 2020. Re-use permitted under CC BY. Published by BMJ.)