Allergic diseases were recently reported related to both SARS-CoV-2 infection and severe COVID-19 through observational studies. However, their causal relationship remains unclear. Herein, we performed univariate and multivariate Mendelian randomization (MR) studies to investigate the causal association between various allergic diseases and COVID-19. Genome-wide association studies (GWAS) summaries were used in this study, with 360838 participants in the allergic disease database, 455449 in asthma, 217914 in allergic rhinitis (AR), and 205764 in atopic dermatitis (AD). In univariate MR analysis, the allergic disease was not causally in connection with SARS‐CoV‐2 infection but have a causal protective effect on severe COVID-19. In contrast, no significant causal effect was found of SARS-CoV-2 infection/severe COVID-19 on the allergic disease. AD, mixed asthma, and childhood asthma were causal protective factors for severe COVID-19. Multivariate MR analysis further revealed the dominant role of asthma in allergic diseases and the dominance of childhood asthma in asthma subtypes. In summary, this study, based on the population genetic variation model, pointed out the protective effect of allergic diseases against severe COVID-19, which may provide some inspiration for further exploration of the pathophysiological mechanisms of COVID-19.