In a statement by the Foreign Ministry, the African country accused Qatar of having connections to terrorist organisations.
It said Qatar is "promoting extremist thoughts and spreading chaos and disturbances across many of the Arab countries, resulting in big humanitarian miseries."
Qatar long has denied supporting terror groups.
Mauritania has strong military and economic ties to oil- rich Saudi Arabia, which led the diplomatic spat with Qatar.
Saudi Arabia, Bahrain, the United Arab Emirates, Egypt and others have severed diplomatic relations and cut off air, land and sea access to Qatar, in the most serious Gulf diplomatic crisis since the 1991 war against Iraq.