Super Saiyan 100 is the absolute most powerful form that a Saiyan can reach. Obtaining this form is extremely difficult, as the only known way to obtain it is to train for one's absolute peak for 5 full years in the Super Saiyan 10 form. Upon the completion of the 5 years, the Saiyan immediately transforms. Goku was the only Saiyan to achieve this form, by travelling into the Hyperbolic Time Chamber, intending to train in preparation for the arrival of the Galaxy Destroyers on Earth, and then ended up escaping with Instant Transmission after transforming (since the exit had long since vanished), as he felt the power of the form was more then enough to achieve victory. It has black fur.
While this form would technically be called Super Saiyan 11, Kibito Kai states that the form is "far too powerful to merit just a one number increase", and begins referring to it as Super Saiyan 100 instead.
While this form is reached following Super Saiyan 10, it does not resemble that form at all, and instead resembles previous forms much more closely. The reason for this is unknown.