An association has been consistently found between diabetes mellitus and decreased lung function. We evaluated to what extent low-grade inflammation (as measured by the level of high-sensitivity C-reactive protein [hs-CRP]) could explain this relationship. A sample of 1878 middle-aged adults from the cross-sectional Enquête Littoral Souffle Air Biologie Environnement survey without self-reported pulmonary and atherosclerosis disease was included. A mediation analysis was performed to assess and quantify the hs-CRP level as a mediator of the relationship between diabetes and lung function. Diabetes was associated with higher hs-CRP level (+22.9%, 95% confidence interval = [5.1, 43.6]). The hs-CRP (>4 vs. ≤1 mg/L) was associated with lower percentage predicted values for the forced expiratory volume in the first second (FEV1) (-4% [-6.1, -1.9]) and forced vital capacity (FVC) (-4.4% [-6.5, -2.3]). Diabetes was associated with FEV1 (-3.5% [-5.8, -1.3]) and FVC (-3.6% [-5.9, -1.3]). The proportion of the effect that is mediated by hs-CRP was 12% [2.4, 37] and 13% [3.7, 39.4] for FEV1 and FVC, respectively. Our results suggest that low-grade systemic inflammation could only explain a small part of the relationship between diabetes and lung function.