The number of people providing informal care has increased considerably in the last years while, at the same time, about one in four Australians have financial stress problems. This study uses rich longitudinal data from the Household, Income and Labour Dynamics in Australia (HILDA) survey to estimate the effect of informal care on financial stress. To establish causality, we exploit a fixed effect-instrumental variable approach to address omitted variable bias and reverse causality problems. Our findings show that informal caregiving increases financial stress between 9.9 and 14.5 percentage points. This finding is robust across a battery of quasi-experimental methods. The effect of informal caregiving on financial stress is more pronounced among males, rural residents and those living in low socioeconomic areas. Our analyses further show that financial fragility and social isolation are important channels through which informal caregiving affects financial stress.