Introduction - Social inequalities in health are a major public health issue. In the field of cancer, the literature highlights social disparities in incidence and mortality. However, no study has yet analyzed social inequalities in survival among a large sample of cancer patients in France. Objective - To study the influence of the social environment on the survival of cancer patients, using data from the French Network of Cancer Registries (Francim). Methods - Approximately 210,000 cases of cancer diagnosed between 2006 and 2009, recorded in 21 registries and followed up through June 30, 2013, were included. Socioeconomic environment was measured using the European Deprivation Index (EDI). Analysis of net survival (i.e., survival adjusted for other possible causes of death) was based on the Pohar-Perme method and flexible modeling of excess mortality rates. Results - Age-standardized 5-year net survival was lower among individuals living in the most deprived areas for 14 out of 16 solid tumors in men and 16 out of 18 solid tumors in women, with differences in magnitude varying by cancer type. Among men, 5-year net survival was reduced by 6.4 percentage points for colorectal cancer, 3 percentage points for prostate cancer, and 2.9 percentage points for lung cancer among the most disadvantaged. Among women, net 5-year survival was reduced by 5.5 percentage points for colorectal cancer, 5.1 percentage points for breast cancer, and 3.6 percentage points (not statistically significant) for lung cancer among the most disadvantaged. The results were more nuanced for hematologic malignancies. Modeling confirmed a significant effect of the social environment on survival for all solid tumors (except sarcomas and thyroid cancer), and for Hodgkin’s lymphoma, four non-Hodgkin’s lymphomas, and chronic myeloproliferative syndromes. Cancer-related excess mortality could be up to twice as high among patients in the most disadvantaged areas compared to patients in the least disadvantaged areas (e.g., melanoma in men, chronic lymphoid leukemias, or bile duct cancers in women). Conclusion - This study reveals a unidirectional social gradient in survival for nearly all cancers in France, with poorer survival among patients living in the most disadvantaged areas.