Nowadays, climate change is a severe issue that is attracting global attention. Effective forest monitoring is vital among various methods to cope with climate change. Unfortunately, current approaches to forest monitoring are costly and sophisticated, making them difficult to apply widely or affordable for developing countries. In this paper, we propose a cost-effective Internet of Things (IoT) system called EcoSentry, which is low-cost and straightforward but still provides reasonably accurate, real-time data for efficient forest monitoring. The proposed system is comprised of two main components: a data-collecting hardware and a data-processing software. The first component includes multiple sensing nodes managed by a single gateway. Each node can collect various data such as temperature, soil moisture, air humidity, and rain level, then transfer them to the gateway via a 2.4 GHz wireless connection. Meanwhile, the second component utilizes Firebase, Angular, and Google Maps platforms for a web interface and Java, Google OAuth, and Google Maps platforms for an Android application on mobile devices. Experimental results show that the proposed system can efficiently support multi-point real-time monitoring and automatically trigger alerts to warn relevant authorities in emergent cases. Furthermore, the proposed system is scalable and can easily integrate with other systems.