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Pulmonary hypertension caused by fibrosing mediastinitis (PH-FM) is a high-risk condition with limited validated tools for prognostic assessment. Although diffusing capacity of the lung for carbon monoxide (DLCO) predicts outcomes in several cardiopulmonary diseases, its prognostic value in PH-FM remains unclear. In this multicenter retrospective cohort study, we included 106 patients with confirmed PH-FM from 14 centers across China. The primary endpoint was all-cause mortality. Cox proportional hazards models were used to assess the association between baseline percent-predicted DLCO and mortality, with sequential adjustment for clinical, hemodynamic, and spirometric variables. The final cohort included 106 patients, of whom 60 had DLCO ≥ 60% predicted and 46 had DLCO < 60% predicted. Over a median follow-up of 36 months (interquartile range [IQR], 14.0-50.8), 27 deaths occurred. Across Cox models, each 10-percentage-point increase in percent-predicted DLCO was associated with a 30% lower risk of all-cause mortality. DLCO < 60% predicted was associated with higher all-cause mortality compared with DLCO ≥ 60% predicted, with hazard ratios ranging from 6.90 to 7.80 across models. Lower baseline percent-predicted DLCO was independently associated with a higher risk of all-cause mortality in patients with PH-FM. DLCO may provide a simple, noninvasive measure to complement current risk assessment in this population.