<p>Community-based conservation has gained traction in the Brazilian Amazon because of its potential in combining territorial protection, local well-being and biodiversity conservation. We assessed the footprint of effective protection, areas actively monitored and defended through community-led surveillance, where illegal activities such as poaching, fishing and logging are successfully prevented, within the largest community-based fisheries conservation arrangement in the Amazon. While the arrangement focused specifically on 13 lakes which were on average 47 ha in size, the effectively protected floodplain area was approximately eightfold larger than the extent of direct protection, defined as the immediate focal area sustaining financial returns through co-management. The additional protection of this ‘functional area’ was on average 11,188 ha, or 36-fold larger than the directly protected area. Although the average cost of effective protection was low (US$0.95 ha⁻¹ yr⁻¹), this was entirely incurred by low-income local communities. Our study underscores the remarkable effort leveraged by Amazonian rural communities in protecting natural ecosystems and the imperative need to develop compensation mechanisms to financially reward them, which are currently lacking.</p>