Excessive generalization of fear and avoidance are hallmark symptoms of chronic pain disability, yet research focusing on the mechanisms underlying generalization of avoidance specifically, is scarce. Two experiments investigated the boundary conditions of costly pain-related avoidance generalization in healthy participants who learned to avoid pain by performing increasingly effortful (in terms of deviation and force) arm-movements using a robot-arm (acquisition). During generalization, novel, but similar arm-movements, without pain, were tested. Experiment 1 (N=64) aimed to facilitate generalization to these movements by reducing visual contextual changes between acquisition to generalization, whereas Experiment 2 (N=70) aimed to prevent extinction by increasing pain uncertainty. Both experiments showed generalization of pain-expectancies and pain-related fear. However, Experiment 2 was the first and only to also demonstrate generalization of avoidance, i.e. choosing the novel effortful arm-movements in the absence of pain. These results suggest that uncertainty about the occurrence of pain may prolong recovery, due to reduced disconfirmation of threat beliefs when exploring, resulting in persistent avoidance. Perspective: This article demonstrates generalization of instrumentally acquired costly pain related avoidance in healthy people under conditions of uncertainty. The results suggest that targeting pain-related uncertainty may be a useful tool for clinicians adopting a psychological approach to treating excessive pain-related avoidance in chronic pain.