Abstract Background A previous study showed an association between CD4 T-cell count decline in people with human immunodeficiency virus infection (PWH) with viral suppression and an increased risk of severe morbid conditions. We aimed to assess the risk of CD4 T-cell count decline (hereafter, CD4 decline), determine associated factors, and evaluate the association of this decline with the risk of severe morbid conditions (cardiovascular disease and cancer) or death. Methods From the Agence Nationale de Recherches sur le SIDA et les hépatites virales (ANRS) CO4 French Hospital Database on HIV cohort, we selected PWH >18 years old who had been followed up for ≥2 years after viral suppression following the initiation of combination antiretroviral therapy (cART) between 2006 and 2018. CD4 decline was defined as 2 consecutive relative differences ≥15%. Among participants with such decline, we modeled CD4, CD8, and total lymphocyte counts before and after CD4 decline, using spline regression. The remaining objectives were assessed using Poisson regression, with the association between CD4 decline and the risk of severe morbid conditions or death evaluated during or after 6 months of decline. Results Among 15 714 participants (75 417 person-years), 181 presented with CD4 decline (incidence rate, 2.4/1000 person-years (95% confidence interval, 2.1–2.8). CD8 and total lymphocyte counts also showed a similar decline. Older current age and lower viral load at treatment initiation were associated with the risk of CD4 decline. The risk of severe morbid conditions or death was 11-fold higher during the first 6 months for participants who presented with CD4 decline versus those who did not (incidence rate ratio, 10.8 [95% confidence interval, 5.1–22.8]), with no significant difference after 6 months. Conclusions In PWH with viral suppression, CD4 decline was rare and related to global lymphopenia. It was associated with a higher risk of severe morbid conditions or death during the first 6 months.