The study examined the effect of self-perceived loneliness on cognitive functioning and self-perceived cognitive ability in 510 healthy older adults (354 women) aged 64 to 103 years (mean 78.78 ± 8.72 years). Loneliness was assessed using the UCLA Loneliness Scale-3, cognitive performance using the standardized GEMS screening, and self-perceived cognitive effectiveness using the SMAC tool. The results showed that higher perceived loneliness is associated with worse cognitive performance (B = -1.18; p < 0.001) and lower self-perceived cognitive efficacy (B = 2.48; p < 0.001). Loneliness leads to an underestimation of one's own cognitive abilities even with average performance (F = 9.75; p < 0.001). Even with well-preserved cognitive efficiency, loneliness causes an underestimation of personal cognitive resources.