Smart agriculture, driven by the Internet of Things (IoT), has emerged as a transformative approach to enhancing agricultural productivity, resource efficiency, and environmental sustainability. This study provides a comprehensive and integrative review of IoT-based precision agriculture systems, focusing on their technical architecture, performance outcomes, and practical implementation feasibility. The analysis demonstrates that IoT-enabled smart irrigation systems can reduce water consumption by 25–50%, while maintaining or increasing crop yields by 10–20%, depending on environmental and operational conditions. Furthermore, integration of multi-sensor networks, artificial intelligence (AI), and cloud–edge computing significantly improves decision accuracy, reduces input waste (fertilizers and pesticides), and enhances real-time responsiveness to environmental stress. The findings confirm that IoT-based systems provide measurable agronomic, economic, and environmental benefits rather than purely conceptual advantages. A layered technical framework is synthesized to illustrate how sensing, communication, data processing, intelligence, and actuation layers form a cyber-physical agricultural system. In addition, the study evaluates key challenges related to scalability, interoperability, infrastructure readiness, and economic feasibility, highlighting pathways for large-scale deployment. Overall, the results demonstrate that IoT-enabled precision agriculture represents a viable and data-driven solution for improving water-use efficiency, farm profitability, and sustainable food production under increasing climatic and resource constraints.