The continuing decline in catch rates instigates various fishing adjustments to keep up with the demands of a growing population. Fishery resources are being caught before they can attain their optimum harvestable size. Undersized catch elicits lower economic value; thus, considered as losses in postharvest fisheries. The present study focused on generating actual data on the volume of undersized catches in selected landing sites in the Philippines. It aims to quantify the magnitude of postharvest and financial losses incurred from catching fishery commodities below their marketable sizes. The estimated loss at 0.97% and 4.02% for capture and aquaculture commodities, respectively, was equivalent to PHP 15,235,290 financial loss. Estimation of losses by commodity showed that squid recorded the highest at 20.14%, followed by tilapia (9.61%), blue swimming crab (4.48%), shrimp (2.75%), small pelagics (1.98%), mussel (1.46%), oceanic tuna (0.91%), by-catch (0.79%), milkfish (0.09%), and oyster (0.02%). Excessive catching of undersized BSC and squid in Western Visayas may lead to overexploitation of resources and may adversely affect subsequent recruitment in the long run. The study's results indicate that catching undersized species could lead to substantial postharvest losses and subsequent loss of potential revenue to the industry players. Allowing the stocks to attain their maximum biomass level will minimize postharvest losses; thus, maximizing utilization of resources and benefits derived from the sector. Unrestrained catching of undersized fishery commodities undermines resource sustainability, economic potential, and food security. The strengthening of regulatory frameworks is, therefore, necessary to address both economic and ecological impacts.