The Williamson Diamond Mine (also known as the Mwadui mine) is a diamond mine 23 kilometres (14 mi) northeast of Shinyanga in Tanzania; it became well known as the first significant diamond mine outside of South Africa. The kimberlite pipe was found in March 1940 and the mine established by John Williamson, a Canadian geologist, and has been continuous operation since then, making it one of the oldest continuously operating diamond mines in the world. Over its lifetime it has produced over 19 million carats (3,800 kg) of diamonds. A 2020 report by The Guardian said that high-quality pink diamonds from the mine could value up to $700,000 a carat.

The Williamson mine was once owned by its namesake Williamson and later nationalized by the government of Tanzania. Since February 2009 the mine is mostly owned by Petra Diamonds, with 75% ownership, the government of Tanzania owning the remaining 25%. In 2020, the mine came under scrutiny because of alleged human rights violations.

The Williamson diamond mine is a large open pit mine currently about 90 meters (300 ft) deep. Diamond mining operations at the Williamson diamond mine are composed of four distinct activities: mining of the pit, re-treatment of tailings to recover missed diamonds, and gravel mining both on the property and adjacent to the property where gravel has been alluvially deposited. The open pit mining and tailings re-treatment are the largest of the four operations. The mine employs about 1,100 staff, mostly Tanzanians.

The mine is located about 160 kilometers (99 mi) south of the town of Mwanza in Kishapu District of Shinyanga Region. Williamson, the discoverer of the site, first owner and namesake of the mine, named the site "Mwadui" after a local chief; "Williamson" and "Mwadui" are now virtually synonymous in the diamond mining world.

The most important geological feature of the Williamson diamond mine is the kimberlite pipe on which it is located. At 146 hectare in area at surface level, it ranks as the largest economically exploitable diamond-bearing volcanic pipe in the world. Recent exploratory drill cores conducted by De Beers have indicated that the pipe is a pyroclastic kimberlite, not a hypabyssal kimberlite as earlier suspected. This indicates that it may be possible to extend the current 90-meter deep open pit to as much as 350 meters deep, and continue down even further with underground mining operations. However, a review of mine operations and long term plans continues.

The Williamson diamond mine is now characterized by low ore grade of about 6 carats (1,200 mg) per hundred tons (12 mg/t) of ore. This is a dramatic decrease from its production in the mine's early life; during the first 25 years of operation, average ore grade was about 30 carats (6.0 g) per hundred tons (60 mg/t); in the first few years of full operation grades were as high as 62 carats (12.4 g) per hundred tons (124 mg/t), or 10 times the ore realized today. Production in the 1950s and 1960s was usually between 500,000 and 750,000 carats (100 and 150 kg) per year; the peak year of production was 1966, when 924,984 carats (185 kg) were produced. Today production levels for all Williamson mine diamond recovery activities is about 300,000 carats (60 kg) per year.

Notable stones produced at the Williamson mine include the 54.5 carats (10.90 g) Williamson pink diamond which was presented to then-Princess Elizabeth and Prince Philip upon their wedding in 1947, and a 388 carats (77.6 g) diamond found in 1990.

The diamond industry of Tanzania in the 1930s was characterized by a scattering of very small mining operations that were insignificant in the economic makeup of the country.