Underground Ernie is a 2006 British computer animated children's television series produced by Joella Productions for the BBC on both CBeebies and BBC Two, and sold internationally by BBC Worldwide. It is set in International Underground, a fictional worldwide underground railway network, based on the London Underground, and focuses on the everyday adventures of Ernie, a friendly underground supervisor, Millie, his multilingual colleague, and Mr. Rails, the lovable maintenance man.

The locomotives under Ernie's watchful eye all have characters of their own. There is Bakerloo, Victoria, Circle, Jubilee and the twins, Hammersmith & City. From time to time they are joined by their friends from across the world including; Paris, Brooklyn, Moscow, Sydney and Osaka.

The show is aimed at children between the ages of three and eight years old. The initial (one and only, so far) series had twenty six episodes, lasting twelve minutes each. In October 2005, the series was officially announced, with it premiering the following year. Its first showing on was on 5 June 2006 on BBC Two at 9:00am. It later premiered on CBeebies until 23 December 2006.