Human
Like all hanyō, InuYasha turns completely human once a month. During the new moon his hair turns black, and he loses his supernatural powers, speed and defensive abilities as well as his demonic characteristics, such as his dog ears, claws, fangs, amber eyes and so on. In this form InuYasha can only use Tetsusaiga in its rusty form. Only InuYasha's friends and a few others know his secret as InuYasha tries to stay hidden during the new moon. He returns to his regular form when the sun comes out — he regains all of his powers: claws, fangs, strength, speed, and defensive abilities.

Demon
InuYasha's demon blood takes over anytime his life is in danger and the Tetsusaiga is out of reach. InuYasha first transformed when Naraku's detachment, Goshinki, broke the Tetsusaiga with its fangs. When transformed (pictured right), the white's of InuYasha's eyes turn red, while the other part turns blue with no pupil. Stripes appear on the sides of his face, and his fangs and claws lengthen. Once transformed, he gains a massive boost in all of his physical abilities, such as strength and durability, and becomes impervious to pain. He cannot, however, distinguish between friend and foe. If left in this state, InuYasha will continue killing until he is killed. Myōga reveals the reason InuYasha's father had the Tetsusaiga forged was to prevent his son's yōkai blood from taking him over since his hanyō mind cannot functionally handle the power he inherited from his father without being overwhelmed.

Purified
While trapped in the Stone Ogre's belly, InuYasha used a shard of the Jewel of the Four Souls to strengthen Tetsusaiga's power, an effort which backfired when the impure powers of the shard flowed through Tetsusaiga, causing InuYasha to transform. Kagome, seeing InuYasha's predicament, ran to him and her embrace purified the shard, leaving InuYasha in a purified version of his yōkai form. In this form, he retained some of the aspects of his yōkai form, such as the purple stripes on his face and the black patch under his eyes, but his eyes remained clear and he retained his sanity instead of being subject to the berserker rage which ordinarily characterizes his yōkai form. His yōki was increased exponentially, allowing him to utilize the Tetsusaiga's Adamant Barrage (fused with the Wind Scar) with an intensity far superior to previous usage. After the shard was removed from the Tetsusaiga, InuYasha reverted to his normal hanyō self.