Centaurea diffusa, also known as diffuse knapweed, white knapweed or tumble knapweed, is a member of the genus Centaurea in the family Asteraceae. This species is common throughout western North America but is not actually native to the North American continent, but to the eastern Mediterranean.

Diffuse knapweed is an annual or biennial plant, generally growing to between 10 and 60 cm in height. It has a highly branched stem and a large taproot, as well as a basal rosette of leaves with smaller leaves alternating on the upright stems. Flowers are usually white or pink and grow out of urn-shaped heads carried at the tips of the many branches. Diffuse knapweed often assumes a short rosette form for one year, reaching maximum size, then rapidly growing and flowering during the second year. A single plant can produce approximately 18,000 seeds.

It is native to Eastern Europe and Western Asia, specifically the nations of Turkey, Syria, Bulgaria, Greece, Romania, Ukraine, and southern Russia.

Diffuse knapweed is considered an invasive species in some parts of North America, having established itself in many areas of the continent. C. diffusa was first identified in North America in 1907 when it was found in an alfalfa field in Washington state. The seeds had presumably been transported in an impure alfalfa seed shipment coming from somewhere in the species native range. Now present in at least 19 states in the United States, it has naturalized in all contiguous states west of the Rockies and additionally in Connecticut, Massachusetts, and New Jersey. Portions of western Canada have also been colonized by this plant.

Areas in which diffuse knapweed has been established generally are plains rangelands or forest benchlands. Land that has recently been disturbed is commonly colonized. It grows in semi-arid and arid environments and seems to favor light, dry, porous soils. Areas with large amounts of shade or high levels of water discourage diffuse knapweed growth.

C. diffusa can be dispersed in multiple ways, such as contamination of food, wind dispersal, and water dispersal however wind is the primary dispersal method.

By 1998 diffuse knapweed had naturalized over 26,640 square kilometres (10,290 sq mi) in the western US, and was increasing its range at a rate of 18% annually. Diffuse knapweed can establish itself in grassland, scrubland and riparian environments. It has little value as feed for livestock, as its thistles can damage the mouth and digestive tract of animals that attempt to feed on it. A study in 1973 concluded that ranches lost approximately US$20/km² (8 cents per acre) of diffuse knapweed due to decreased grazing area. In an agricultural setting, it can greatly reduce crop yield and purity. ^{[citation needed]}

Effective control of diffuse knapweed requires a fusion of well-executed land management, biological control, physical control, chemical control and reestablishment of the native species. Any method of control must ensure that the root is removed or the plant will grow back. Additionally, native plant growth in areas where diffuse knapweed has been removed should be encouraged to prevent reestablishment.