Archibald McKellar (3 February 1816 – 11 February 1894) was briefly leader of Canada's Ontario Liberal Party from 1867 to 1868 and, unofficially, the first Leader of the Opposition in Ontario's new provincial legislature (though he was not officially recognised as such) and went on to serve as Commissioner of Public Works in Ontario Premier Oliver Mowat's first government.
He was born near Inveraray, Scotland in 1816 and came to Upper Canada with his parents in 1817. He helped on the family farm in Kent County and was also part owner of a large sawmill at Chatham. He served in the militia during the Upper Canada Rebellion, becoming a major in the Kent Battalion. In 1857, he was elected to the Legislative Assembly of the Province of Canada for Kent and he served until Confederation. He was defeated by Rufus Stephenson when he ran in Kent in the 1867 election, but he was elected to the 1st Parliament of Ontario for Bothwell in 1867 and 1871. He married a second time to Mary Catherine Powell in 1874.