Objectives Physical activity (PA) habit strength influences how people control their PA and may also influence how people encode, store, or recall their PA. This study evaluated whether individual differences in PA habit strength moderated the magnitude of within-person associations between weekly self-reported PA and step counts. Design Longitudinal. Method After an initial assessment of PA habit strength, university students wore pedometers for four weeks and completed four self-reports of weekly mild, moderate, and strenuous PA. Results On average, people's weekly step counts and self-reported PA across time were weakly-to-moderately associated, but there was substantial variability in the magnitude of these associations across people. People with strong PA habits had weaker within-person associations between deviations from their average self-reported PA and step counts than those with weak PA habits. Conclusions These results may indicate that PA habit strength influences the sensitivity of self-report PA measures to change in objectively-measured PA.