Growing evidence shows that conservation strategies such as fishing bans may adversely affect the livelihoods of low-income communities, which often encourages community members to fish illegally. In this study, we determined the underlying factors that influenced fishermen to engage in illegal fishing during the closed season in the largest lake of Bangladesh, Kaptai Lake. Out of 231 fishermen interviewed, 46.8% were engaged in illegal fishing during the closed season. Engagement in illegal fishing was positively related to receipt of a financial loan and negatively related to receipt of a government subsidy and secondary occupation of the fishermen. Financial gain by selling the fish was the prime motive for illegal fishing. Surprisingly, fishermen received a 36% lower price for illegally harvested fish during the closed season (mean price ± SD, BDT 95.8 ± 64.9), when compared to that during the open fishing season (BDT 151.5 ± 64.9). Mid-level actors secured 99%-154% more profit during the closed season. This study shows that multiple factors are important for the engagement of fishermen in illegal fishing, and therefore, an integrated conservation policy considering all causative factors should be employed to achieve more effective conservation in the region's inland waters.