Background: Cancer is a leading cause of morbidity and mortality globally. Dyspnea, affecting up to 60% of cancer patients, exacerbates physical and psychological distress, reducing quality of life. This study aims to explore the relationship between dyspnea and factors such as age, sex, clinical diagnosis, and treatment lines in cancer patients, with the goal of improving understanding and management of this debilitating symptom to enhance patient care and quality of life. Methods: This study employed an observational, cross-sectional, and descriptive approach to investigate patients with oncological disease at the University Hospital of Salamanca between March 2021 and April 2024. A convenience sample was selected, including patients over 18 years old with a pathological diagnosis of cancer, experiencing any degree of dyspnea, and who consented to participate by signing the informed consent. Exclusion criteria included lack of consent and clinical conditions that prevented an interview. The studied variables encompass sociodemographic (age, gender, diagnosis, tumor stage, number of treatment lines) and clinical aspects (daily activities, degree of dyspnea, functional capacity, physical performance), evaluated using the Barthel Index, the mMRC Dyspnea Scale, the ECOG Scale, and the Short Physical Performance Battery (SPPB). Data were collected through semistructured interviews and medical records, and analyzed using specialized software. This research has ethical approval CEiM Code 2023 12 1472, Reference 2024/01. Results: The mean age was 66.82 years. Lung cancer was predominant (60.2%), with most patients in stage 3 (65.7%) and receiving three treatment lines (68.7%). Higher age, advanced disease stage, and more treatment lines correlated with lower Barthel and SPPB scores, and higher ECOG and mMRC scores, indicating worse functionality, physical performance, and greater dyspnea. No significant correlations were found between gender or pathological diagnosis and the studied variables. Conclusions: Advanced age, higher disease stage, and more treatment lines are associated with decreased functionality, poorer physical performance, and increased dyspnea in cancer patients. Gender and specific cancer diagnosis do not significantly affect these relationships. Addressing dyspnea is crucial to improving the quality of life and physical performance in this population. Future studies should explore additional factors like treatment types and nutritional status.