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The discovery of new viruses currently outpaces our capacity for experimental examination of infection biology. To better couple virus discovery with immunology, we genetically modified zebrafish to visually report on virus infections. After generating a strain that expresses green fluorescent protein (GFP) under an interferon-stimulated gene promoter, we repeatedly observed transgenic larvae spontaneously expressing GFP days after hatching. RNA sequencing of GFP-positive zebrafish revealed a picornavirus distantly related to known viruses. We detected this virus in publicly available sequencing data from seemingly asymptomatic zebrafish in research institutes world-wide. These data revealed widespread tissue tropism and identified the clonal CG2 strain as notably vulnerable to picornavirus infection. Infection induces the expression of numerous antiviral immune defense genes in zebrafish, an effect also evident in public data. Our study describes a prevalent picornavirus that infects zebrafish, and provides new strategies for simultaneously discovering viruses and their impact on vertebrate hosts.