Helm Hammerhand (Third Age 2691 – 2759, aged 68 years) was the ninth King of Rohan and last King of the first line.

Helm became King at the death of his father Gram, during a time of crisis. His father had already fought open warfare with the Dunlendings, who operated from the captured fortress of Isengard.

In T.A. 2754 a man named Freca, a Dunlending with Rohirric blood planned to have his son, Wulf, marry the daughter of Helm. Freca rode to Edoras with a great force of men, planning to threaten the King to comply with his demands, but Helm Hammerhand smote him with his fist and Freca died soon thereafter. Because of his great strength Helm was known as Hammerhand.

The men of Freca left Edoras, but in 2758 they returned with a great force of Dunlendings, reinforced by enemies of Gondor who had landed in the south. Under the leadership of Wulf they overran Rohan; Helm fought them at the Crossings of Isen but lost and withdrew into the Suthburg and there endured a long siege.

Helm held the fortress during the Long Winter. He blew his great war-horn, and broke through the Dunlending ranks, clad in white, stalking men like a snow-troll slaying them with his bare hands. However, his sons Haleth and Háma were slain and Helm grew gaunt because of grief and famine; still his horn would fill the Dunlendings with fear each time it was heard. During one of his night sorties Helm died, apparently from famine and cold; his body was discovered frozen in the snow still standing and ready to fight.

He was succeeded by his nephew Fréaláf Hildeson, who defeated the Dunlendings.

There was a belief that his horn was sometimes heard in the Deep and that his wraith still wandered through Rohan, and defended the keep that would be renamed the Hornburg.