As population aging and digital transformation continue worldwide, the Internet is playing an increasingly important role in addressing age-related mental health challenges. However, evidence on the relationship between Internet use and mental well-being among middle-aged and older adults across different countries is limited and inconsistent. Leveraging data from six aging cohorts across 23 high- and middle-income countries, we used linear mixed models and meta-analyses to examine the longitudinal association between baseline Internet use and subsequent mental health outcomes among adults aged ≥50 years (n=87,803). Independent of confounders, Internet use was associated with fewer depressive symptoms (pooled β = -0.09, 95% confidence interval [CI] = -0.12 to -0.07), higher life satisfaction (pooled β = 0.06, 95% CI = 0.03 to 0.09), and better self-reported health (pooled β = 0.15, 95% CI = 0.13 to 0.17). Cross-national differences in the beneficial effect of Internet use were observed and could be partially explained by country-level factors, such as gross domestic product per capita and world happiness index. The protective role of Internet use varied in different subpopulations within a country and was identified across people with different genetic risks. A higher frequency and an additional wave of Internet use were also related to improved mental health. These findings could provide significant implications for public health policies and practices in promoting mental well-being in later life through the Internet. Internet use can provide great benefits for mental health of older adults, especially in countries with limited Internet access and mental health services.