Introduction: The first case of COVID-19 in Sweden was diagnosed in late January 2020, the first recommendations against the spread of the virus were released in mid-March, and the peak of the first wave of the pandemic was reached in March-June. The aim of this cross-sectional study was to assess the short-term effects of the first wave of the COVID-19 pandemic on prostate cancer (PCa) diagnosis, staging, and treatment. Materials and methods: Data in the National Prostate Cancer Register (NPCR) of Sweden on newly diagnosed PCa cases and on the number of diagnostic and therapeutic procedures performed between 18 March 2020 and 2 June 2020 were compared with those in the corresponding time periods in 2017-2019, as reported until January 31 of the year after each study period. Results: During the study period in 2020, 36% fewer PCa cases were registered in NPCR compared with the corresponding time period in previous years: 1458 cases in 2020 vs a mean of 2285 cases in 2017-2019. The decrease in new PCa registrations was more pronounced in men above age 75 years, down 51%, than in men aged 70-75, down 37%, and in men below age 70, down 28%. There was no decrease in the number of radical prostatectomies and number of radical radiotherapy courses increased by 32%. Conclusions: During the peak of the first phase of the COVID-19 pandemic, the number of men diagnosed with PCa in Sweden decreased by one third compared with previous years, whereas there was no decrease in the number of curative treatments.