Niels (Latin: Nicolaus, Engish exonym Nicholas; c. 1065 – 25 June 1134) was the King of Denmark from 1104 to 1134. Niels succeeded his brother Eric Evergood and is presumed to have been the youngest son of King Sweyn II Estridson. King Niels actively supported the canonization of Canute IV the Holy and supported his son Magnus I of Sweden by killing the potential successor Knud Lavard. His secular rule was supported by the clergy. Niels was killed in an ensuing civil war, and his successor was Eric II Emune.

Niels was born around 1063, married in 1105, and died in 1134. His parents were King Sweyn II Estridson and an unknown concubine. Four of Niels' brothers ascended the throne before him. He is first mentioned in history in 1086 when he was sent to replace his brother, King Olaf I, in the town of Flanders. When his brother Eric Evergood died in July 1103 during a pilgrimage, Niels, who was the last surviving brother, was elected king the following year ahead of Evergood's chosen deputy Harald Kesja. He married Margaret Fredkulla, the daughter of Inge I of Sweden. She was believed to have wielded considerable influence during his reign.

During the majority of Niels' reign, the country had internal peace. The contemporary Chronicon Roskildense describes him as mild and forthcoming, though not a competent ruler. He carried through reforms of his personal hird retinue, separating it into a military and an administrative branch responsible for tax collection. He placed family members as his jarls, including his son-in-law Jarl Ubbe and his nephews Eric (Emune) and Canute Lavard, who were the sons of Eric Evergood. Niels supported the canonization of his brother Canute IV the Holy by bestowing gifts on the clergy of Odense as he sought to enlarge the power and influence of the monarchy through the aid of the church. However, it is likely he did not win the full support of the magnates on Zealand. He also campaigned against the Wends in alliance with Poland. In 1125, Niels' son Magnus was named King of Sweden as Magnus I.

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