Diabetes is one of the most common chronic diseases worldwide. The study aimed to present an epidemiological analysis of hospitalization related to diabetes mellitus in Poland between 2014 and 2020 as well as to analyze changes in diabetes-related hospital admissions before and during the COVID-19 pandemic. This study is a retrospective analysis of the national registry dataset of hospital discharge reports on diabetes-related hospitalizations in Poland between 2014 and 2020. The number of diabetes-related hospitalizations varied from 76,220 in 2016 to 45,159 in 2020. The hospitalization rate per 100,000 has decreased from 74.6 in 2019 to 53.0 in 2020 among patients with type 1 diabetes (percentage change: −28.9%). An even greater drop was observed among patients with type 2 diabetes: from 99.4 in 2019 to 61.6 in 2020 (percentage change: −38%). Both among patients with type 1 and type 2 diabetes, a decrease in hospitalization rate was higher among females than males (−31.6% vs. −26.7% and −40.9% vs. −35.2% respectively). When compared to 2019, in 2020, the in-hospital mortality rate increased by 66.7% (60.0% among males and 65.2% among females) among patients hospitalized with type 1 diabetes and by 48.5% (55.2% among females and 42.1% among males) among patients hospitalized with type 2 diabetes. Markable differences in hospitalization rate, duration of hospitalization, as well as in-hospital mortality rate by gender, were observed, which reveal health inequalities.