Background: Adolescents’ perceptions, attitudes, and lived experiences regarding cancer diagnosis significantly influence their quality of life and treatment adherence. To inform the formulation of tailored interventions, we explored these factors among adolescents receiving care at a sub-Saharan African health facility. Methods: We conducted a qualitative study from July 2022 to December 2022, at Mbarara Regional Referral Hospital in western Uganda. In-depth interviews were held with 30 adolescents aged 10–17 years who had been diagnosed with cancer. Using NVivo 12 software, a codebook and coding framework were developed to generate themes aligned with study objectives. Ethical approval was obtained from the Research and Ethics Committee of Mbarara University of Science and Technology. Results: Participants had a median age of 13.5 years; 19 were male. Diagnoses included leukemia (13), lymphoma (10) solid tumors (7). Initial. perceptions and attitudes towards their diagnosis were predominantly negative but improved over time as they received information and treatment. Perceptions and attitudes were poorer among those responding poorly to treatment and those who had had extremely negative experiences. Negative experiences included body disfigurement, social challenges, emotional distress, physical pain, and interrupted education. Positive experiences included improvement in symptoms and support from health workers and their families. Conclusion: Adolescents initially exhibit poor perceptions and attitudes towards their cancer diagnosis, which tend to improve with treatment and support. Their experiences are mixed, highlighting the need for specialized education and counselling services to address knowledge gaps, reduce negative attitudes, and improve overall care outcomes.