Seafood mislabeling has numerous consequences, including economic deception and food safety risks. The focus of this study was to investigate fish species labeling, use of acceptable market names, and Country of Origin Labeling (COOL) compliance for fresh fish fillets sold at grocery store seafood counters in Southern California. A total of 120 fillets representing 16 different categories of fish were collected from 30 Perishable Agricultural Commodities Act (PACA)-listed grocery stores. Each sample underwent DNA barcoding to identify the species. Acceptable market names were confirmed using the FDA Seafood List. Samples were determined to be compliant with COOL if both the country of origin and the production method were declared in accordance with regulatory requirements. Species substitution was detected in 16 of the 120 samples (13.3%) and unacceptable market names were observed for an additional 11 samples (9.2%). The highest rates of species substitution were recorded for snapper (3/3), yellowtail (2/4), halibut (4/10), cod (3/10), and bass (2/7). COOL noncompliance was observed for 28 samples (23.3%): the country of origin was missing for 15 samples, production method was missing for 9 samples, and 4 samples were missing both. When all forms of mislabeling were considered, 47 of the 120 samples (39.2%) had at least one labeling error. The majority of grocery stores (25/30) had one or more samples with a mislabeling error. This study revealed species mislabeling as a continuous concern in the seafood industry, especially with higher-valued species. Furthermore, the lack of COOL compliance among retailers is concerning and suggests a need for increased focus on these regulations.