A systematic review and meta-analysis investigated the association between body mass index (BMI) and prognosis in patients with interstitial lung disease (ILD). We analyzed 25 studies with 23,741 patients, mostly cohort, and searched PubMed/MEDLINE, Embase and Cochrane Library databases until May 2024. Obesity was associated with lower mortality (RR = 0.91, 95% CI = 0.87–0.94, I² = 0%) and higher BMI inversely with mortality (HR = 0.94, 95% CI = 0.92–0.96, I² = 56%). Obese patients had lower baseline forced vital capacity (FVC; MD = -1.61, 95% CI = -3.10 to -0.12, I² = 64%) but higher baseline diffusing capacity for carbon monoxide (DLCO; MD = 1.85, 95% CI = 0.84–2.85, I² = 42%). They had a smaller decrease in FVC (9% decrease in MD and a slower decrease of 6.6%). Higher BMI may be associated with lower mortality and better lung function in patients with ILD.