Zephyr (/ˈzɛf ər/) is a small real-time operating system (RTOS)[7] for connected, resource-constrained and embedded devices (with an emphasis on microcontrollers) supporting multiple architectures and released under the Apache License 2.0.

Zephyr includes a kernel, and all components and libraries, device drivers, protocol stacks, file systems, and firmware updates, needed to develop full application software.

[11] Compared to Wind River's other RTOS, VxWorks, Rocket had much smaller memory needs, especially suitable for sensors and single-function embedded devices.

[15] As of January 2025[update], Zephyr had the largest number of contributors and commits compared to other RTOSes (including Mbed, RT-Thread, NuttX, and RIOT).

[19] Also, being owned and supported by a community means the world's open source developers are vetting the code, which significantly increases security.