BACKGROUND: Cancer is a leading cause of premature mortality globally. This study estimates premature deaths at ages 30-69 years and distinguishes these as deaths that are preventable (avertable through primary or secondary prevention) or treatable (avertable through curative treatment) in 185 countries worldwide. METHODS: For this population-based study, estimated cancer deaths by country, cancer, sex, and age groups were retrieved from the International Agency for Research on Cancer's GLOBOCAN 2020 database. Crude and age-adjusted cancer-specific years of life lost (YLLs) were calculated for 36 cancer types. FINDINGS: Of the estimated all-ages cancer burden of 265·6 million YLLs, 182·8 million (68·8%) YLLs were due to premature deaths from cancer globally in 2020, with 124·3 million (68·0%) preventable and 58·5 million (32·0%) treatable. Countries with low, medium, or high human development index (HDI) levels all had greater proportions of YLLs at premature ages than very high HDI countries (68·9%, 77·0%, and 72·2% vs 57·7%, respectively). Lung cancer was the leading contributor to preventable premature YLLs in medium to very high HDI countries (17·4% of all cancers, or 29·7 million of 171·3 million YLLs), whereas cervical cancer led in low HDI countries (26·3% of all preventable cancers, or 1·83 million of 6·93 million YLLs). Colorectal and breast cancers were major treatable cancers across all four tiers of HDI (25·5% of all treatable cancers in combination, or 14·9 million of 58·5 million YLLs). INTERPRETATION: Alongside tailored programmes of early diagnosis and screening linked to timely and comprehensive treatment, greater investments in risk factor reduction and vaccination are needed to address premature cancer inequalities. FUNDING: Erasmus Mundus Exchange Programme and the International Agency for Research on Cancer. TRANSLATIONS: For the German, French, Spanish and Chinese translations of the abstract see Supplementary Materials section.