The molecular mechanism behind the process of biodegradation and consequently the loss in mechanical properties of polylactic acid (PLA) requires detailed understanding for the successful designing of various technological devices. In this study, we examine the role of free water and chain scission in this degradation process and quantify the mechanical properties of pristine and nanoparticle-reinforced PLA as it degrades over time. The in situ mechanical response of the degrading polymer is determined experimentally using nano-dynamic mechanical analysis (nanoDMA). Water present in the polymer matrix contributes to hydrolysis and subsequent scission of polymer chains. Water in excess of hydrolysis, however, alters the load transfer mechanism within the polymer chains. Molecular mechanism study applied in this work provides detailed insights into the relative role of these two mechanisms, (i) chain scission and (ii) solvation, in the reduction of mechanical properties during degradation. Functional groups such as ester (-COO-) and terminal acid (-COOH) interact with water molecules leading to the formation of water bridges and solvation shells, respectively. These are found to hinder the load transfer between polymer chains. It is observed that, compared to scission, solvation plays a more active role in the reduction of mechanical properties of degrading PLA. © 2018 American Chemical Society.