Buk, Ulsan (Korean: 울산광역시 북구) is a constituency of the National Assembly of South Korea.

The constituency was established ahead of the 2000 South Korean legislative election from the former Ulju, Ulsan and Jung, Ulsan constituencies.

Unlike its neighboring constituencies, Buk, Ulsan is considered a swing constituency as it has elected representatives from progressive, liberal, and conservative parties.

This swing characteristic can partially be attributed to the fact that a substantial portion of the constituency's voters are blue collar workers, who traditionally have a stronger tendency to vote for progressive and liberal parties.

[1] The constituency encompasses the neighborhoods of Nongso-dong, Gangdong-dong, Hyomun-dong, Songjeong-dong, Yangjeong-dong, and Yeompo-dong.