The Italian spring goby (Orsinigobius punctatissimus) is a critically endangered freshwater fish endemic to the Padanian plain, inhabiting springs and resurgences. Over recent decades, habitat loss, pollution, and climate change have driven a range contraction, fragmenting populations and reducing the availability of reproductive substrates. Springs and resurgences are additionally exposed to increasing plastic contamination, raising questions about whether plastic debris may functionally substitute for natural nesting substrates. During a monitoring campaign, we repeatedly observed O. punctatissimus using macroplastic items as nest sites, suggesting opportunistic exploitation of these wastes. Building on these observations, the present study was aimed at 1) assessing whether the species shows a preference for specific macroplastic types and 2) evaluating whether the provision of artificial substrates could enhance reproductive success. To address the first aim, we systematically collected 193 macroplastics across ten reproductive sites during peak breeding season, categorizing each by type and recording egg presence. Hunting shotshells, whose small, enclosed cavity structurally resembles natural nesting sites, were disproportionately selected for oviposition relative to their availability. To address the second aim, 134 hollow bricks were deployed at eight springs. Fish density increased by 970% following brick placement, resulting in values 1348% higher than those of control sites. Small holes were preferentially used, especially when the invasive red swamp crayfish (Procambarus clarkii) was abundant, suggesting a trade-off between nest size and protection from predators and competitors. These findings highlight the novel use of macroplastics as opportunistic nests, suggesting that cavity structure drives substrate selection. Furthermore, targeted artificial substrates offer an effective conservation strategy to overcome nest-site limitations and buffer the negative effects of invasive species on reproduction.