A dictatress or a dictatrix is a female dictator.
There have been very few female dictators in recent history. Many female absolute monarchs have exercised dictatorial powers, but female military dictators or coup instigators have been virtually non-existent.
The term dictatress is also often used to describe the dictatorial actions of a non-human, such as a country. For example, President John Quincy Adams once stated his fear that the United States was in danger of becoming "dictatress of the world."