For most publishers, yes, you need an agent. But why wouldn't you want one? An agent results in higher advances, as well as publications in different languages and movie rights. They do the legwork and make sure you both get what your book's worth.
I know querying agents is a pain and terribly stressful, but it really does help. Take the time that's needed to find an agent. You'll also learn a lot more about the publishing industry if you do.