A psychoeducational intervention (Keep your brain fit!) was designed for the middle-aged and older working population. The intervention focuses on increasing knowledge and awareness about cognitive ageing and teaching strategies to cope with cognitive changes. The primary aim of this study was to investigate the effectiveness of the e-health intervention in terms of subjective cognitive functioning. As secondary aims, objective cognitive functioning and psychological well-being were also measured. A randomised controlled trial that included people aged 40 to 65 years was conducted. A maximum of 4 weeks was allowed to complete the intervention. The outcome measures were obtained from an online test battery that was administered at baseline, post-test and at 4-week follow-up. A total of 376 participants completed the whole study. After the intervention, the experimental group reported more feelings of stability concerning memory functioning and perceived greater locus of control over memory compared to the control group. These effects were maintained at the 4-week follow-up. Taking into account the relatively low costs and easy accessibility of this e-health intervention, we consider the programme to be a valuable contribution to public healthcare interventions for middle-aged and older adults.