Apple Watch OS (watchOS)

Source: http://www.apple.com.© 2015 Apple Inc. License: All Rights Reserved.

San Francisco Compactwas designed specifically for the Apple Watch. In the same genre as Neue Helvetica, which has long been Apple's choice for system fonts, this typeface has larger apertures, flatter sides, and looser spacing, making for a more legible experience overall, especially for the small, glanceable type on a watch display.

Source: http://www.apple.com.© 2015 Apple Inc. License: All Rights Reserved.

Press images for watchOS 2.

Source: http://www.apple.com.© 2015 Apple Inc. License: All Rights Reserved.

Source: http://www.apple.com.© 2015 Apple Inc. License: All Rights Reserved.

San Francisco has a vertically-centered contextual alternate colon that appears when placed between numerals.

© 2015 Apple Inc. License: All Rights Reserved.

The Activity app uses a special rounded version of San Francisco.