Did Apple create an entirely new typeface for the Apple Watch? Did they customize something existing? To me it looks like it was inspired by DIN and given a very tall x-height, but I see influence from Roboto in there as well.
Whatever it is, it’s grotesque.
Seriously though, DIN does look close but for its square tittles and punctuation marks and not getting the 9’s right.
I fancy Aktiv Grotesk for a closer match on the numbers, but it’s just not ‘square’ enough and again lacks round tittles, etc.
UPDATE: Apple did indeed create their own typeface for the font: http://www.apple.com/watch/technology/#familiar
We even developed a new typeface to maximize legibility.
According to the Apple Watch HIGs, it's a new font called San Francisco, and it comes in two flavors:
Choose the San Francisco Text font for text that is 19 points or smaller. Choose the San Francisco Display font for text that is 20 points or larger.