Augmented reality (AR) is a promising technology for building applications in an Internet of Things (IoT) environment, utilized for visualizing information provided by IoT devices. In this paper, we enable Web-based mobile AR applications with mobile agents in a resource-oriented IoT system architecture. We present an adaptable mobile agent composition that contains the data representation logic and mappings between AR applications and system resources. Thus, mobile agents and AR application-specific data structures are exposed as global system resources. System resource linkages are considered between real-world objects and their virtual representations for mobile agent-based AR applications. The agent composition also complies with the REST principles for resource access and control system-wide. This allows dynamic runtime adaptation and addressing the device and resource heterogeneity, thus eliminating the need for application-specific communication protocols. Moreover, we utilize a Web-based mobile AR application framework, running completely in a Web browser, which facilitates straightforward AR application development. Lastly, a proof of concept mobile AR application is implemented, where a coffee maker with a visual tag is connected to a low-power resource-constrained wireless sensor network node as an IoT device. A mobile agent is injected into the IoT environment to expose the state changes of the coffee maker. Through the visual tag, AR applications are able to visualize the state changes of the coffee maker in their user interface.