The current study aimed to examine whether the Geneva Space Cruiser – a new online adaptation of the Cruiser – represents a valid, reliable and useful tool to assess prospective memory (PM) across the adult lifespan via fully self-administered online testing. Therefore, an adult lifespan sample of 252 adults (19–86 years old) performed the Geneva Space Cruiser in the laboratory and online, at home, and also performed a more traditional laboratory PM task. A second sample of 224 young adults (19–35 years old) participated in a test-retest online assessment of the Geneva Space Cruiser. Bayesian analyses showed that the Geneva Space Cruiser yielded similar results when administered in the laboratory versus online, both in terms of data distribution as well as of key outcome measures (i.e., PM performance and monitoring). Results further showed very good test-retest reliability and acceptable construct validity. Finally, the online tool was sensitive for detecting age-differences similar to those typically observed in laboratory studies. Together, our findings suggest that the Geneva Space Cruiser represents a rather valid, moderately to highly reliable, and generally useful tool to assess PM in online testing across wide ranges of the adult lifespan, with certain limitations for the oldest participants and for women.