Background: The number of deaths because of cancer among adolescent and young adults (AYAs) in Japan is about 2,300 per year, which accounts for 17.3% of all deaths in these populations. This study aimed to investigate the clinicopathological features of AYA cancer patients. Methods: Patients registered to the Ibaraki Cancer Registry from our hospital between 2007 and 2017 were included in this study. The proportion of patients by cancer types were calculated by age groups and gender. Results: There were 27281 cancer patients in the registry, among whom, 1972 patients (7.2%) were aged between 15 and 39 years (AYA group) and 2445 were aged between 64 and 66 years (reference group). Female patients were the majority (68.2%) in the AYA group, whereas male patients were the majority (59.7%) in the reference group. Among male patients, central nervous system (22.3%) and germ cell (22.3%) tumors were the most common types in the AYA group, whereas prostate (14.7%) and lung (11.3%) cancers were the dominant ones in the reference group. Among female patients, cervical (35.8%) and breast (14.5%) cancers were the top two cancer types in the AYA group, whereas breast (19.3%) and lung (8.2%) cancers were the dominant ones in the reference group. The proportion of cancer types by 5-year age bands showed three distinctive patterns that could be associated with their etiology. Pattern 1 included central nervous system neoplasm, ovarian cancer, hematopoietic malignancy, bone sarcoma, soft tissue sarcoma, and germ cell tumor, in which the proportion peaked in teenage year group and gradually decreased in the older age groups. Pattern 2 included thyroid, breast, cervix, and uterine cancers, as well as malignant melanoma and lymphoma, which peaked in the twenties and thirties. The other cancer types belonged to pattern 3, in which the proportion increased with age and peaked in the sixties. Conclusions: Cancers in AYAs had a distinctive pattern compared to that in older age patients. The proportion of each cancer by age groups showed three different patterns.