In this paper, a novel complementary metal–oxide–semiconductor fish spoilage detector is proposed as an application of Internet of Things (IoT) in fish markets. The proposed analog processing circuits were successfully used for detecting the freshness of swordfish. The proposed detector can be a low cost and easy-to-use alternative for housewives or for IoT applications in fish markets. Moreover, the detector can be directly delivered through transmission media of IoT devices because of its digitized output. The functions and performance of the proposed detector were evaluated and validated through measurements. The measured resistance of the proposed chip ranged from 20 $\text{k}\Omega $ to 1 $\text{M}\Omega $ , and the corresponding output period range was 6.21– $313~\mu \text{s}$ . The maximum linear error was 0.53%, and the sensitivity was 0.313 $\mu \text{s}/\text{k}\Omega $ . The chip area was measured to be $1.26\times 1.24$ mm2. The freshness detection time for the swordfish was 0–50 min, and the corresponding output period range was 107.51– $14.66~\mu \text{s}$ . The freshness of the fish could thus be easily determined based on the output period of the proposed chip. The proposed chip is suitable for IoT applications for detecting fish spoilage, such as remotely detecting fish spoilage in fish markets.