Ultra Super Saiyan (超サイヤ人第3段階|, Sûpâ Saiya-jin dai san-dankai; lit. "Super Saiyan 3rd Stage") is the second branch of advanced Super Saiyan, achieved through intense training in the Super Saiyan form. This form is not related to the third branch of Super Saiyan (Full-Power Super Saiyan). Like the Ascended Super Saiyan stage, it is a forced increase in the power output of the initial transformation, and thus it is not itself an actual transformation. All natural inhibitions of the body are completely removed in this state, resulting in 100% utilization of strength and stamina reserves. It is important to note that "Ultra Super Saiyan" is a fan-made term, but has widely been accepted as a proper name for the form.
In this stage, the Saiyan is similar in appearance to the Legendary Super Saiyan form. The skin of the Saiyan may become slightly redder due to increased blood flow, and muscle size is increased to the point of practical ridiculousness. The aura no longer flows upward, but flails outward, with surges of bio-electricity becoming more frequent and prominent.
In the series, the Ultra Super Saiyan form was discovered as an advanced stage of Ascended Super Saiyan (itself an advanced stage of the Super Saiyan transformation) that further increased the physical strength of the Super Saiyan form. However, due to this form being so muscular, mass is increased to the point where the user's speed is reduced to debilitating levels. In addition, the Saiyan's energy rapidly depletes due to the effort needed simply to maintain the transformation. Altogether, the negatives of this stage outweigh the positives.
Believing that he surpassed his father Vegeta in strength, Future Trunks ascended to the Ultra Super Saiyan form to combat Perfect Cell at the behest of Vegeta's defeat. Although he managed to hold his own for some time, Future Trunks was also unable to defeat Cell because of his inability to maintain equality in speed. Because of his inexperience with these transformations, Future Trunks did not realize that his bulky form would be an inhibitor of his ability to battle, a characteristic that was later noted by both Goku and Cell. Future Trunks assumed that Vegeta was aware of this disadvantage as well, believing this to be the reason Vegeta didn't use the Ultra Super Saiyan form.
Perfect Cell displayed a similar ability to bulk up his muscles when he mocked Future Trunks for using the Ultra Super Saiyan form. When Perfect Cell later fought Gohan in his Super Saiyan 2 form, he ended up losing his temper and resorted to using his slow, Power Weighted form, which failed to help him win the battle.
In Dragon Ball Z: Supersonic Warriors 2, this form is known as "Super Saiyan 1.5" and can only be used as Super Saiyan Future Trunks' ultimate move, whereas he simply uses its massive power output to fire a powerful Finish Buster and reverts back as soon as the move is complete.
In Dragon Ball Z: Budokai, Future Trunks in his Ultra Super Saiyan is called "Super Trunks 2" (Super Trunks being Future Trunks in his Ascended Super Saiyan from).
The Budokai Tenkaichi and Raging Blast series both include this form as "Super Trunks", complete with its flaws of slow movement and a high tension boost ability called "Inexperienced Power-Up".