Conjugation to polyethylene glycol (PEG) is a widely used approach to improve the therapeutic value of proteins essentially by prolonging their body residence time. PEGylation may however induce changes in the structure and/or the stability of proteins and thus on their function(s). The effects of PEGylation on the thermodynamic stability can either be positive (stabilization), negative (destabilization), or neutral (no effect). Moreover, various factors such as the PEG length and PEGylation site can influence the consequences of PEGylation on the structure and stability of proteins. In this study, the effects of PEGylation on the structure, stability, and polymerization of alpha1‐antitrypsin (AAT) were investigated, using PEGs with different lengths, different structures (linear or 2‐armed) and different linking chemistries (via amine or thiol) at two distinct positions of the sequence. The results show that whatever the size, position, and structure of PEG chains, PEGylation (a) does not induce significant changes in AAT structure (either at the secondary or tertiary level); (b) does not alter the stability of the native protein upon both chemical‐ and heat‐induced denaturation; and (c) does not prevent AAT to fully refold and recover its activity following chemical denaturation. However, the propensity of AAT to aggregate upon heat treatment was significantly decreased by PEGylation, although PEGylation did not prevent the irreversible inactivation of the enzyme. Moreover, conjugation to PEG, especially 2‐armed 40 kDa PEG, greatly improved the proteolytic resistance of AAT. PEGylation of AAT could be a promising strategy to prolong its half‐life after infusion in AAT‐deficient patients and thereby decrease the frequency of infusions.