Protected areas (PAs) remain the primary conservation instrument of Madagascar’s unique but threatened biodiversity. We combine matching and panel regressions in a quasi-natural experiment setting to analyze PAs’ environmental effectiveness annually between 2001 and 2012 and study two channels that moderate the impact: initial poverty rates and local variations in law enforcement. Our findings show that PAs have stabilized deforestation around a positive trend without having halted it. Their overall environmental impact is however limited: PAs created before the 2000 have helped to slow down deforestation by approximately 20%, meaning that 80% of forests are still cleared even though they are protected. As for new PA created from the mid-2000s, the early impact is statistically not significant. As a result, the total welfare impact of protection is currently uncertain. We show that PAs have been effective for municipalities where overall law enforcement was the lowest: PAs have helped to limit what we call opportunistic deforestation. Meanwhile, PAs have been poorly effective when poverty rates were high: when necessity is the driver of deforestation, PAs are not sufficient to slow down deforestation. As a consequence, effectively stopping deforestation in Madagascar will require ambitious policies to trigger the necessary agricultural transition for the country.