Saniniu Laizer earned £2.4m ($3.4m) from the country's mining ministry for the gemstones, which had a combined weight of 15kg (33 lb).
"There will be a big party tomorrow," Mr Laizer, a father of more than 30 children, told the BBC.
Tanzanite is only found in northern Tanzania and is used to make ornaments.
It is one of the rarest gemstones on Earth, and one local geologist estimates its supply may be entirely depleted within the next 20 years.
The precious stone's appeal lies in its variety of hues, including green, red, purple and blue.
Its value is determined by rarity - the finer the colour or clarity, the higher the price.
Mr Laizer mined the stones, weighing 9.2kg and 5.8kg, last week, but he sold them on Wednesday during a trading event in the northern region of Manyara.
Until now the largest Tanzanite rock to be mined weighed 3.3kg.
President John Magufuli phoned in to congratulate Mr Laizer on the find.
"This is the benefit of small-scale miners and this proves that Tanzania is rich," the president said.
