Scent-detection dogs have proven ability in terrestrial biosecurity and conservation, but limited evaluation to detect aquatic targets. Invasive fish are extremely difficult and costly to eradicate, so early detection methods are crucial. Current methods are resource-intensive and inadequate at low abundances. This study evaluated dogs’ ability to detect an aquatic pest, common carp (Cyprinus carpio). Other invasive species were used to test dogs’ discrimination: bullhead catfish and goldfish. Water samples were taken with carp present (positive), and aquaria containing no fish, catfish, or goldfish, respectively (negative samples). Samples were placed inside an automated apparatus and dogs evaluated samples using a go/no-go procedure. Completion criteria for dogs included achieving ≥80% accuracy across all samples for four sessions. All dogs detected carp successfully, with an average sensitivity of 97.5% and specificity of 92.3%. Samples were then diluted and three dogs detected carp at dilutions equivalent to biomass of 37.4 kg/ha (a density of biological significance in terms of environmental damage). Dogs also successfully discriminated between carp and the other fish species. This research demonstrates that dogs can detect fish in water samples. This novel system has potential as an effective and economically-viable alternative for the detection of invasive aquatic species.