<p>This systematic review and meta-analysis evaluated whether narrowband ultraviolet B (NB-UVB) phototherapy affects serum folate levels. A systematic search of PubMed, Web of Science, and Embase, supplemented by manual searching, identified self-controlled clinical trials that examined the impact of NB-UVB phototherapy on serum folate levels. Nine studies met the inclusion criteria, all of which underwent rigorous screening and quality assessment. Using random-effects models, we calculated standardized mean differences (SMDs) and 95% confidence intervals (CIs), and further performed subgroup analyses, meta-regression, sensitivity analyses, and publication bias assessments with funnel plots and Egger’s test. The pooled analysis indicated that NB-UVB phototherapy was associated with decreased serum folate levels, although the reduction was not statistically significant (SMD = -0.56, 95% CI: -1.19 to 0.06, <i>P</i> = 0.077). Similar non-significant decreases were observed in psoriasis subgroup (SMD = -0.79, 95% CI: -1.63 to 0.05, <i>P</i> = 0.065) and the vitiligo subgroup (SMD = -0.72, 95% CI: -1.56 to 0.11, <i>P</i> = 0.089). Meta-regression analyses did not identify any significant moderators contributing to between-study heterogeneity. Sensitivity analyses confirmed the robustness of the overall pooled result, while exclusion of certain studies in the psoriasis and vitiligo subgroups led to significant decreases in serum folate levels and substantially reduced heterogeneity. No substantial publication bias was detected. Further well-designed studies with larger sample sizes and higher methodological quality are still needed to validate these findings.</p>