Objective: To analyze the efficacy and safety of nintedanib in patients with fibrosing autoimmune disease–related interstitial lung diseases (ILDs) with a progressive phenotype. Methods: The INBUILD trial enrolled patients with a fibrosing ILD other than idiopathic pulmonary fibrosis, with diffuse fibrosing lung disease of >10% extent on high-resolution computed tomography, forced vital capacity percent predicted (FVC%) ≥45%, and diffusing capacity of the lungs for carbon monoxide percent predicted ≥30% to <80%. Patients fulfilled protocol-defined criteria for progression of ILD within the 24 months before screening, despite management deemed appropriate in clinical practice. Subjects were randomized to receive nintedanib or placebo. We assessed the rate of decline in FVC (ml/year) and adverse events (AEs) over 52 weeks in the subgroup with autoimmune disease–related ILDs. Results: Among 170 patients with autoimmune disease–related ILDs, the rate of decline in FVC over 52 weeks was −75.9 ml/year with nintedanib versus −178.6 ml/year with placebo (difference 102.7 ml/year [95% confidence interval 23.2, 182.2]; nominal P = 0.012). No heterogeneity was detected in the effect of nintedanib versus placebo across subgroups based on ILD diagnosis (P = 0.91). The most frequent AE was diarrhea, reported in 63.4% and 27.3% of subjects in the nintedanib and placebo groups, respectively. AEs led to permanent discontinuation of trial drug in 17.1% and 10.2% of subjects in the nintedanib and placebo groups, respectively. Conclusion: In the INBUILD trial, nintedanib slowed the rate of decline in FVC in patients with progressive fibrosing autoimmune disease–related ILDs, with AEs that were manageable for most patients.