The authorities found the drugs while investigating the smuggling of more than 600 kg of cocaine discovered in May.
That was seized after a small plane made an emergency landing at the northern port of Nouadhibou, police said on Monday.
Ely Ould Sneiba, director of the country's anti-drugs office, said: "During our investigations we noticed a parked minibus which looked different from other such vehicles even though it had registration plates from this country."
UN drugs officials estimated that cocaine was worth around $42,000 per kg on arrival in Europe during 2005, which would put the wholesale value of Monday's seizure at well over $30m.
Law-enforcement officials say Guinea Bissau, which adjoins Mauritania's southern neighbour Senegal, is used as a hub by Latin American cartels to traffick drugs to Europe.
A fortnight ago Senegal incinerated nearly 2.5 tonnes of cocaine seized in June after a deserted sailing yacht loaded with the drug drifted into a popular resort.
Mauritania is sparsely populated, has a long Atlantic coastline and stretches deep into the Sahara, making it "a perfect backdrop" for trafficking, Kissy Agyeman, a London-based Africa analyst for risk consultancy Global Insight, says.
"Security controls in Mauritania remain a problem for curbing the illicit entry of drugs, often bound for European destinations, because vast swathes of desert coastline are unmanned."