Background Increasing evidence suggests that alterations in gut microbiota are associated with a variety of skin diseases. However, whether this association reflects a causal relationship remains unknown. We aimed to reveal the causal relationship between gut microbiota and skin diseases, including psoriasis, atopic dermatitis, acne, and lichen planus.Methods We obtained full genetic association summary data for gut flora, psoriasis, atopic dermatitis, acne, and lichen planus from public databases and used three methods, mainly inverse variance weighting, to analyze the causal relationships between gut flora and these skin diseases using bidirectional Mendelian randomization, as well as sensitivity and stability analysis of the results using multiple methods.Results The results showed that there were 5 associated genera in the psoriasis group, 10 associated genera were obtained in the atopic dermatitis group, a total of 8 associated genera in the acne group, and 6 associated genera in the lichen planus group. The results corrected for false discovery rate showed that Eubacteriumfissicatena(p = 7.10E-05, OR = 1.44,95%CI: 1.20–1.72) and Lactococcus(p = 4.90E-04, OR = 1.37,95%CI: 1.15–1.65) and psoriasis, and Coprococcus-3(p = 0.001, OR = 2.39,95%CI: 1.41–4.03) and acne still showed a causal relationship. In contrast, in the reverse Mendelian randomization results, there was no evidence of an association between these skin diseases and intestinal flora.Conclusion We demonstrated a causal relationship between gut microbiota and immune skin diseases and provide a new therapeutic perspective for the study of immune diseases: targeted modulation of dysregulation of specific bacterial taxa to prevent and treat psoriasis, atopic dermatitis, acne, and lichen planus.