Apple will reportedly make their new San Francisco typeface the system font for both iOS 9 and OS X 10.11.

iOS 9 and OS X 10.11 may take a typography lesson from the Apple Watch

San Francisco, the current system font on the Apple Watch, will reportedly also be the system font for both iOS 9 and OS X 10.11. It would replace Helvetica Neue, the current system font on both iOS and OS X. San Francisco has reportedly been in testing as the font of choice since March, according to 9to5Mac:

Multiple Apple employees tell us that new daily builds of OS X 10.11 and iOS 9 began including the new font toward the end of March. Apple will likely also push developers to redesign their apps ahead of the new font's general release this fall.

San Francisco is in heavy use in one of Apple's iPhone apps already. The Activity app, which appears on iOS 8.2 or later after you pair an Apple Watch, primarily utilizes the font to display your activity details. Apple is also using it as the new keyboard font on their latest MacBook Some have also already taken a crack at bringing San Francisco to OS X Yosemite.

Source 9to5Mac