Open Sans is an open source humanist sans-serif typeface that was designed by Steve Matteson under commission from Google. It was released in 2011 and is based on his earlier design called Droid Sans, which was specifically created for Android mobile devices but with slight modifications to its width.

The typeface is characterized by its wide apertures on many letters and a large x-height, making it highly legible on screens and at small sizes. Being part of the humanist genre of sans-serif typefaces, it also features a true italic style. As of July 2018, Open Sans is the second most widely used font on Google Fonts, serving over four billion views per day across more than 20 million websites.

In March 2021, the Open Sans font family was updated to include a variable font version, which now also supports Hebrew characters.

Open Sans is popular in flat design-style web design.

Open Sans is used in some of Google's web pages as well as its print and web advertisements. It is the official font of the UK's Labour, Co-operative, and Liberal Democrat parties.

Used in WordPress 3.8 which was released on December 12, 2013.

According to Google, it was developed with an "upright stress, open forms and a neutral, yet friendly appearance" and is "optimized for legibility across print, web, and mobile interfaces." Its design is similar to that of Matteson's Droid Sans, created as the first user interface font for Android phones, but with wider characters and the inclusion of italic variants. Explaining the different name, Matteson has said "Droid was intentionally narrow for mobile screens but it didn't have 'narrow' in the name. Open Sans isn't really 'extended' so that wasn't really an option either. A new family name is the direction they chose."

The character repertoire contains 897 glyphs, covering the Latin, Greek and Cyrillic alphabets with a wide range of diacritics.