Which macOS version was the first to support the Apple File System (APFS)?

The first macOS version to support APFS was macOS High Sierra, also known as macOS 10.13. Apple released it on September 25, 2017, introducing the Apple File System as the successor to HFS+ for Mac storage.

APFS was designed for modern flash and solid-state storage, with features including copy-on-write metadata, snapshots, space sharing, strong encryption, and fast directory sizing. On High Sierra, Macs with flash storage were generally converted to APFS during installation. Fusion Drives and traditional hard drives received broader automatic conversion support later, with macOS Mojave.

macOS Sierra 10.12.4 did contain an experimental, limited APFS implementation accessible through command-line tools. That preview is why Sierra is sometimes incorrectly named as the first supported version. High Sierra was the first full macOS release to provide production APFS support and make it the default file system for appropriate Mac installations.