This Recommendation specifies the SensorThings application programming interface (API) which provides an open standard-based and geospatial-enabled framework to interconnect Internet of things (IoT) devices, data, and applications over the Web. The SensorThings API is an open standard, and that means it is non-proprietary, platform-independent. It builds on a rich set of proven-working and widely-adopted open standards, such as the Web protocols and the Open Geospatial Consortium (OGC) sensor Web enablement (SWE) standards, including the ISO/OGC observation and measurement data model. The SensorThings API is extensible and can be applied to not only simple but also complex use cases. This Recommendation provides a standard way to manage and retrieve observations and metadata from heterogeneous IoT sensor systems. The SensorThings API uses representational state transfer (REST) principles, an efficient JavaScript object notation (JSON) encoding, message queuing telemetry transport (MQTT) protocol, flexible OASIS open data protocol (OData) and uniform resource locator (URL) conventions.