The original Apple Watch used a chip called the S1.
Apple described the S1 as a system-in-package rather than merely a standalone processor. It combined the application processor with memory, storage, wireless connectivity, power management, and controllers for sensors and input/output in one sealed package. This compact arrangement helped Apple fit a complete computer into the watch's small case.
The first-generation Apple Watch arrived in April 2015 and used a single-core 520 MHz 32-bit processor. It included 8GB of storage, though watchOS limited how much could be used for music and photos. Unlike later models, it had no built-in GPS and relied on a paired iPhone for location services.
The S1 is often confused with the S2, which powered Apple Watch Series 2. Apple also later used the name S1P for Series 1; that chip was a faster dual-core design related to the S2, not the original S1.