In retirement saving, many people miss out on early opportunities to save and subsequently fail to take adequate actions for a long time thereafter. We examined whether these two observations—the initial failure to act and the subsequent inertia—could be related through the phenomenon of inaction inertia. In Experiment 1 (N = 180), participants were less likely to save for retirement when the difference in annual return between the current opportunity and the missed opportunity was large versus small. In Experiment 2 (N = 180), participants were less likely to start saving for retirement when reminded of a missed opportunity 10 years ago versus 1 year ago. These data constitute the first demonstration of inaction inertia in retirement saving: People’s reluctance to act on attractive saving opportunities may be induced by their previous inaction. In Experiment 3 (N = 340) and Experiment 4 (N = 628), we find that the observed inertia is the product of a tendency to underestimate exponential growth combined with a focus on past opportunities. Building on this mechanism, Experiment 5 (N = 916) provided evidence for a potential remedy; the inaction inertia effect completely disappeared when focus was shifted from required contributions to future outcomes.