Niels Arntzen Sem (1782-1859) was a Norwegian politician.
He was elected to the Norwegian Parliament in 1825 and 1827, representing the constituency of Stavanger Amt. He worked as district stipendiary magistrate (sorenskriver) there.
In 1828 he was appointed County Governor of Buskeruds Amt. While stationed here, he was elected to the Norwegian Parliament in 1830. He moved on to be County Governor of Akershus Amt and Christiania Stiftamt in 1831 and County Governor of Lister og Mandals Amt in 1837. While stationed there, in Christianssand, he was to the Norwegian Parliament in 1842 and 1845.
In 1830 and 1845 he was President of the Storting, sharing the position with several people.
From 1846 to 1857 he was diocesan county governor of County Governor of Christiania Stiftamt. From October 1852 to 12 April 1853 he was an acting member of the Council of State Division in interim in Stockholm, founded during King Oscar I's illness.