What is the name of the first Apple Watch to include an always-on display?

The first Apple Watch with an always-on display was the Apple Watch Series 5.

Apple introduced the Series 5 in September 2019 with an Always-On Retina display. Unlike earlier Apple Watch models, its screen could remain visible when the user's wrist was lowered, allowing the time and selected watch-face information to be read without raising the wrist or tapping the display.

The feature depended on a low-temperature polycrystalline oxide, or LTPO, OLED display, a low-power display driver, improved power management, and an ambient light sensor. To preserve battery life, the watch dimmed the display when the wrist was down and returned it to full brightness when raised or tapped. Apple rated the Series 5 for up to 18 hours of battery life.

Series 4 is a frequent near-miss because it introduced a larger display and major health features, but it did not have an always-on screen. Series 6 continued the feature and added faster performance and blood-oxygen sensing.