Prospective memory (PM) is the ability to remember to perform an intended action in the future. In everyday life, people often have contextual information (e.g., the presence of cues) to support the completion of their PM tasks. The present study aimed to investigate how context (as probability of PM cue occurrence over time) and predictability affect PM. In two experiments, participants performed a laboratory PM task having the possibility to check the probability of the next PM cue occurrence whenever they wished; PM cue probability was manipulated to be temporally informative (predictable) or uninformative (unpredictable) on the actual PM cue occurrence. Both experiments showed that PM accuracy and cost on ongoing task performance increased with the presence of contextual information. Experiment 2 showed that this effect was independent of cue focality for PM accuracy but not for PM cost, for which the effect of context was particularly strong for non-focal compared to focal cues. Participants monitored the PM cue with uniform frequency over time, regardless of the context's predictability, and checked the probability of PM cue occurrence more often when the cue was non-focal compared to focal. This study showed the importance of contextual information in PM, highlighting the capacity of people to adapt the allocation of attentional resources systematically over time to optimize strategic monitoring and, in turn, PM performance.