Objective To analyze whether there is a causal association between psoriasis and Alzheimer disease (AD) by a two-sample two-way Mendelian randomization (MR) method. Methods In the forward study, the single nucleotide polymorphisms (SNPs) associated with psoriasis were obtained from the comprehensive statistical data of the genome-wide association study database as the instrumental variables, and AD as the outcome; in the reverse study, the SNPs associated with AD were taken as instrumental variables, and psoriasis as the outcome. Using two-sample two-way MR analysis, the odds ratio (OR) value and 95% confidence interval (CI) of regression models, namely inverse variance weighted (IVW) method, MR-Egger regression method, weighted median method, simple pattern method, and weighted pattern method, were used to evaluate the causal relationship between psoriasis and AD. Cochran’s Q test was used to assess the heterogeneity of genetic instrumental variables, MR-Egger intercept method was used to test the horizontal pleiotropy of the assessment, “leave-one-out” method was used to assess the sensitivity of a SNP to the effect of causality, and the symmetry of funnel plot was observed to assess bias. Results A total of 19 SNPs associated with psoriasis were included as instrumental variables in the forward study. The IVW analysis of the forward study showed that there was a causal correlation between psoriasis and AD [OR=1.032, 95%CI (1.014, 1.051), P<0.001], and MR-Egger regression method [OR=1.042, 95%CI (1.012, 1.073), P=0.013], weighted median [OR=1.048, 95%CI (1.023, 1.074), P<0.001], and weighted model [OR=1.046, 95%CI (1.020, 1.073), P=0.002] all supported this result. Heterogeneity test (IVW result: Q=13.752, P=0.745; MR-Egger regression result: Q=13.134, P=0.727), MR-Egger intercept method (Egger intercept=–0.004, P=0.442), the results of “leave-one-out” method and funnel plot showed that the results of MR analysis were reliable. A total of 127 AD-related SNPs were included as instrumental variables in the reverse study. In reverse research, there was no evidence to support the AD could increase the risk of psoriasis (P>0.05). Heterogeneity test (IVW result: Q=232.496, P<0.001; MR-Egger regression result: Q=232.119, P<0.001) suggested heterogeneity, but MR-Egger intercept method (Egger intercept=0.003, P=0.652), the results of “leave-one-out” method and funnel plot showed that the results of MR analysis were reliable. Conclusion There is a causal association between psoriasis and AD, and psoriasis may increase the risk of AD.