William Cowan Rennie is a Scottish politician who led the Scottish Liberal Democrats from 2011 to 2021. Born in September 1967, he has represented North East Fife in the Scottish Parliament since 2016, having previously served as a list MSP for Mid Scotland and Fife from 2011 to 2016. Before entering Holyrood, Rennie won a by-election to become Member of Parliament for Dunfermline and West Fife in February 2006, though he lost that seat to Labour at the 2010 general election. Between his parliamentary terms, he worked as a Special Government Adviser at the Scotland Office for Liberal Democrat Scottish Secretaries Danny Alexander and Michael Moore.

Following his election to the Scottish Parliament in May 2011, Rennie was elected unopposed as leader of the Scottish Liberal Democrats, succeeding Tavish Scott. He successfully transitioned from a regional list seat to win the North East Fife constituency in 2016 and retained it in 2021, but stepped down as party leader two months after that election, being replaced by Alex Cole-Hamilton. Rennie writes commentary for Comment Central.