<p>The increasing number of IoT devices used in various applications is drawing attention to the Internet of Things (IoT). However, due to its limited processing capabilities, data confidentiality is crucial. Security issues arise due to the constrained nature of IoT systems, allowing intruders to establish connections, inject false data, and launch attacks. Lightweight encryption algorithms are implemented to address this issue but are vulnerable to differential attacks due to their simplicity. This paper proposes a secure, ultra-lightweight 32-bit cipher (SUL32C) for IoT devices. SUL32C is a lightweight block cipher method for IoT networks to protect the confidentiality of message contents. It applies one round of encryption while maintaining the necessary level of security, integrity, and confidentiality services for data. The proposed lightweight cipher utilizes the S-box layer of the SPN structure and integrates it with the ARX structure, a well-known structure of traditional LWC algorithms, to enhance complexity and achieve a higher level of security while minimizing the device's performance cost. Several factors have been used to prove the reliability of any proposed cipher. However, we use randomness tests to demonstrate the efficiency and effectiveness of the pseudo-random number generators in the proposed SUL32C. Additionally, numerous statistical tests demonstrate the efficiency of the proposed SUL32C. We implement the proposed SUL32C on the ESP32 microcontroller. We compare SUL32C with two well-known lightweight cipher algorithms, such as SPECK and SIMON, in terms of security level and performance. The experiment results show that the proposed cipher is faster by 5.13 times, 15.98 times, and 864 times compared to SPECK, SIMON, and PRESENT, respectively. </p>