> Backbone's only hard dependency is Underscore.js ( >= 1.7.0). For RESTful persistence and DOM manipulation with Backbone.View, include jQuery ( >= 1.11.0), and json2.js for older Internet Explorer support. (Mimics of the Underscore and jQuery APIs, such as Lo-Dash and Zepto, will also tend to work, with varying degrees of compatibility.)

It's not a true dependency but it's a default companion and optional dependency (see Backbone.$).

Source: http://backbonejs.org