About 10% of the people infected by the severe acute respiratory syndrome coronavirus 2 are reported to experience “long COVID,” that is, persistence of symptoms several weeks after infection. Day-by-day prevalence of long COVID symptoms was determined from responses to the Long COVID Symptom Tool by 837 patients (3075 person-months) with a confirmed infection and enrolled in the ComPaRe long COVID cohort, a prospective cohort of such patients in France. Nine months after disease onset, 88.7% patients reported persistent symptoms and 70% reported a high burden of disease. Over time, the prevalence progressively decreased for 19/53 symptoms (e.g., loss of taste/smell) and was stable for 29/53 symptoms (e.g., fatigue). For 5/53 symptoms (e.g., memory problems), prevalence increased rapidly over the first two months and then reached a plateau. These findings are important for understanding the underlying etiologies and mechanisms of long COVID.