Background: Even with universal health care, patients living with cancer often face substantial treatment-related costs and income loss in Europe. Insights into the socioeconomic impact of cancer within and across countries are needed to create awareness, inform policy, and develop targeted measurement instruments. The SEC study aims to explore the socioeconomic impact and financial toxicity of cancer and identify vulnerable patient groups across Europe. Patients and methods: To investigate experiences of a large number of patients, data were collected in a collaborative effort of hospitals and patient organizations across Europe through convenience sampling. Patients undergoing treatment currently or treated within the past 2 years could participate. A 44-item survey was developed to measure the socioeconomic impact following a cancer diagnosis. The primary outcome was the level of financial toxicity, measured by the Financial Index of Toxicity (FIT) score. To identify vulnerable groups, multiple regression analyses were used to investigate the association between the FIT score, clinical characteristics, and socioeconomic demographics, including cancer type, employment status, and country of residence. Results: A total of 2507 patients across Europe met the inclusion criteria. Fifty-six percent of the patients reported income loss and 86% additional treatment-related expenses. Sixteen percent of patients delayed or avoided medical visits, buying medication, surgery, or other health services. Next to a significant association of the country of residence, our regression models demonstrated that divorced, self-employed patients who were younger (-0.02; P = 0.000) and lower educated (0.75; P = 0.000) with a lower household income (1.21; P = 0.000) and children (0.21; P = 0.000) at the time of diagnosis reported significantly higher FIT scores compared with older patients who were married (-0.56; P = 0.000), retired (-1.55; P = 0.000), or employed (-0.56; P = 0.000). Conclusions: In every European Union country, a substantial number of patients with cancer report serious financial consequences and stress. Further research is critical to inform well-tailored policies and interventions to limit the socioeconomic impact on patients with cancer.