Apple released watchOS 27.0.1 for Apple Watch Series 12 and Apple Watch Ultra 4 on Sept. 23, with release notes describing bug fixes "including an issue that could cause the Apple Watch to restart unexpectedly," according to MacRumors.
Series 12 and Ultra 4 owners who've noticed their watch rebooting on its own should check for and install watchOS 27.0.1. Apple's notes name that exact symptom among the issues this update addresses. Owners who haven't run into the problem can install it whenever it's convenient.
Apple then expanded watchOS 27.0.1 to all Apple Watch models compatible with watchOS 27 on Sept. 28. Apple now lists watchOS 27.0.1 as the latest watchOS version, while its security releases page records the initial Sept. 23 release for Series 12 and Ultra 4 and the broader Sept. 28 rollout.
watchOS 27 rolled out broadly on Sept. 14, and Series 12 and Ultra 4 became available in stores and online on Sept. 18, according to Apple's launch announcement. watchOS 27.0.1 followed on Sept. 23 for those two models before reaching the rest of the compatible lineup five days later.
Who gets watchOS 27.0.1
watchOS 27.0.1 is now available across the watchOS 27-compatible lineup.
That includes Apple Watch Series 9 and later, Apple Watch SE 3, and Apple Watch Ultra 2 and later. Apple originally limited the Sept. 23 release to Series 12 and Ultra 4, but expanded it on Sept. 28 to the other eligible watches.
The broader watchOS 27 release requires an iPhone 11 or later, or iPhone SE (second generation or later), running iOS 27 or later, per Apple's rollout details.
Series 12 and Ultra 4 both run Apple's new Health Sensing System and S11 chip, which power higher-frequency heart rate and heart rate variability tracking along with a new readiness score, according to Apple's product announcement. Both watches now measure heart rate every five seconds throughout the day, per the same announcement. The cited release information does not connect the restart issue to that hardware.
The two models otherwise differ quite a bit on price and battery. Series 12 offers up to 24 hours of everyday battery life and up to 10 hours during an outdoor workout, and starts at $399, according to Apple's Series 12 announcement. Ultra 4 stretches to roughly two days of everyday use, up to 50 hours, and starts at $799, per Apple's Ultra 4 announcement. Neither figure has any documented connection to the restart bug; they're included here to establish the two watches that received the fix first.
What the update fixes, and what's still undocumented
Apple's published wording for the original Series 12 and Ultra 4 release is limited to what's confirmed: an issue "could cause the Apple Watch to restart unexpectedly," and the update addresses it, per MacRumors' summary of the changelog. "Could cause" is a hedge worth holding onto. It doesn't establish that the restart happens on every unit, or how often, or under what conditions.
When Apple expanded watchOS 27.0.1 to the other compatible models, the release notes described bug fixes without identifying additional specific problems. Apple's security releases page says watchOS 27.0.1 has no published CVE entries.
Those gaps matter for a practical reason. A restart mid-workout could interrupt active tracking, and Ultra 4's Extended Workout mode is built to track up to 25 hours with full GPS and heart rate readings, while its Max Extended Workout mode stretches outdoor runs, walks, and hikes to up to 45 hours, according to Apple's Ultra 4 announcement. That's exactly the kind of long session where an unexpected reboot would be hardest to miss. Apple hasn't said whether the restart bug caused any lost workout data.
Beyond that, the cited release information doesn't say how many watches were affected, what triggers the restart, or whether it tends to happen during workouts, sleep tracking, charging, or general use. Those details are not included in the cited release information, and it doesn't quantify how widespread the issue was.
How to install watchOS 27.0.1
Installation works through the same two paths Apple uses for other watchOS updates. Open Settings on the watch and go to General, then Software Update, or open the Apple Watch app on a paired iPhone and check General, then Software Update, per MacRumors' installation guidance.
The update also requires the watch to be on its charger with a battery level of at least 50 percent. If either condition isn't met, the installation won't proceed.
Given how new Series 12 and Ultra 4 are, some owners may still be setting up watch faces, bands, and Apple Fitness+ trials rather than checking for software updates. Anyone who bought either watch on or after Sept. 18 should add a Software Update check to that setup list.
Who should update
The update is no longer limited to Series 12 and Ultra 4.
Series 12 or Ultra 4 owners who've had a restart happen on its own: check for and install watchOS 27.0.1, since Apple's original notes name this exact symptom as addressed.
Series 12 or Ultra 4 owners who haven't seen the issue: install it whenever convenient.
Series 9, Series 10, Series 11, SE 3, Ultra 2, or Ultra 3 owners: watchOS 27.0.1 is now available for these models as well. Apple describes the broader release as providing bug fixes but does not identify a model-specific restart problem for each of them.
If restarts continue after updating
Apple's original notes tie this release to the unexpected-restart issue on Series 12 and Ultra 4 among unspecified other bug fixes. Apple does not specifically mention battery drain, Bluetooth pairing problems, or third-party app crashes in the cited release information.
If a Series 12 or Ultra 4 keeps restarting after installing watchOS 27.0.1, it's worth noting when the restarts happen and what the watch was doing at the time, then contacting Apple Support with those details. The available sources don't provide independent effectiveness data on how well this patch resolves the issue in practice.
Series 12 and Ultra 4 owners who've experienced unexpected restarts should install watchOS 27.0.1 and contact Apple Support if the problem continues afterward. Owners who haven't experienced the issue can install it at a convenient time. Owners of other watchOS 27-compatible Apple Watches should also now find watchOS 27.0.1 in Software Update following Apple's Sept. 28 expansion.
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