Senescence is progressive, age-related bodily deterioration, accompanied at the population level by declines in average survival and fecundity (i.e., ‘demographic senescence’). Demographic senescence of plants has been investigated in only a few species, including small, floating macrophytes in the genus Lemna (family Araceae, subfamily Lemnoideae – the ‘duckweeds’). Unlike most plant species, Lemna ramets exhibit determinate growth, potentially rendering them more likely to experience demographic senescence. Here, our objective was to investigate senescence in a Lemna species not previously studied in this context, L. gibba L., toward the long-term goal of conducting cross-species comparative analyses. In a longitudinal lab study, we investigated a cohort of 334 individual L. gibba fronds, whose survival and reproduction we followed daily from birth (defined by the date a focal frond detached from its parent) to death (defined by the date a focal frond’s last daughter detached). We fit survival data to exponential, Weibull, Gompertz, and logistic models, the first of which represents ‘no senescence’. The logistic model was found to have the greatest support (AICC weight >0.99), indicating strong age-related declines in survival. We fit reproduction data using a generalized estimating equation approach, which showed a significant age-related decline in the predicted probability of daily reproduction – from 0.61 at age 3 days to 0.23 at age 52 days (i.e., after excluding the first two days of reproduction data to account for the initial, pre-reproductive phase of the L. gibba lifecycle). These age-related declines provide strong evidence that L. gibba does exhibit demographic senescence, consistent with evidence from congeneric species.