Background Reducing meat consumption is advocated for healthier and more sustainable diets. However, behavioral studies are needed to better understand the mechanisms underlying meat-reducing dietary changes. Objective The main aim of this study was to compare the motives associated with stages of change toward meat reduction in French adults, using the transtheoretical model (TTM). A second aim was to investigate the associations between stages of change and adherence to dietary patterns favoring a better balance of animal and plant food consumption over time. Methods This longitudinal study included 25,143 non-vegetarian participants of the web-based NutriNet-Santé cohort with a mean follow-up of 6.2 (SD = 2.6) years. Dietary data were obtained from 24-hour dietary records over the period 2009–2019. Contribution of meat to total energy intake and scores measuring the contribution of healthy and unhealthy plant-based foods to the diet were computed. A questionnaire completed in 2018 allowed us to identify the TTM stages of change related to meat reduction (precontemplation, contemplation, preparation, action, maintenance), and recorded motives related to meat consumption. We used multivariate linear mixed models for repeated data to assess associations between food intake changes and stages, and logistic regression for motives, presented as adjusted frequencies. Results Participants in late stages were characterized by a significantly higher decrease in meat intake over time, compared to the earliest stage (e.g., βmaintenance*time = −0.08, p< 0.0001), and a higher increase in the healthy plant-based food consumption score over time (e.g., βmaintenance*time = 0.11, p< 0.0001). Concerns about health, nutrition, and the environment were the most frequently cited motives for reducing meat consumption at all stages. Conclusions Individuals who had already initiated meat reduction adhered to healthier and more sustainable diets than meat continuers. Characterizing motives according to readiness to reduce meat consumption could support tailored public health campaigns.