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The American shad (Alosa sapidissima) is an important freshwater food fish, yet little is known about its postmortem quality. We sampled the meat of American shad and tracked the changes in color, pH, shear stress, cooking loss and total volatile basic nitrogen (TVB-N) over 48 h of storage at 24 °C (unrefrigerated) or 4 °C (refrigerated). Thereafter, the essential nutrients, fatty acids, hydrolyzed amino acids, free amino acids, and electronic tongue were evaluated. The results show that the L* and a* values decreased as storage progressed, while the b* value increased. The shear force decreased, but cooking losses in the American shad increased; the TVB-N value continuously increased over the duration of storage. The TVB-N content deviated from the freshness range at 48 h when stored at 24 °C. At 24 °C, the sweet amino acids reached a maximum at 6 h, and the bitter amino acids reached a maximum at 48 h. At 4 °C, there was a significant difference in bitter free amino acids at 48 h (p < 0.05). The nutrient composition showed that the contents of fat and protein decreased, whereas the water content increased. These results showed that American shad should be eaten within 6 h when stored at 24 °C, while American shad stored at 4 °C can maintain freshness for 24 h to obtain better product quality.