Kim Yo-jong, who is in her mid to late twenties, was identified as a vice-director of a department within the powerful Central Committee, KCNA said.
Kim Jong-un is the third generation of the Kim family to rule North Korea.
His aunt also had a prominent role in the party but disappeared after her husband was executed for treason.
Observers say could end up also playing a significant supportive leadership role.
She was first seen publically at the funeral of her father, Kim Jong-il, in 2011.
Since then Kim Yo-jong has on occasion been seen in political events and “field guidance trips” accompanying her brother but without any official title in the party.
Kim Jong-un, 31, and his sister are both children of Kim Jong-il and his late wife, Ko Yong-hui.
They are believed to have gone to the same boarding school in Switzerland as their older brother Kim Jong-chol.
Kim Jong-chol does not have any publicly defined role in the government.
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