Struvite (magnesium ammonium phosphate) is a phosphate mineral with formula: NH₄MgPO₄·6H₂O. Struvite crystallizes in the orthorhombic system as white to yellowish or brownish-white pyramidal crystals or in platy mica-like forms. It is a soft mineral with Mohs hardness of 1.5 to 2 and has a low specific gravity of 1.7. It is sparingly soluble in neutral and alkaline conditions, but readily soluble in acid.

Struvite urinary stones and crystals form readily in the urine of animals and humans that are infected with ammonia-producing organisms. They are potentiated by alkaline urine and high magnesium excretion (high magnesium/plant-based diets). They also are potentiated by a specific urinary protein in domestic cats.

Although struvite was briefly mentioned in Hooke's Micrographia, it was first described in detail in 1845 by the German chemist Georg Ludwig Ulex (1811–1883), who found crystals of struvite in what he surmised had once been a medieval midden in Hamburg, Germany; he named the new mineral after the geographer and geologist Heinrich Christian Gottfried von Struve [de] (1772–1851) of Hamburg.

Struvite readily forms in alkaline conditions where its constituent ions are present. In nature, it forms primarily in areas associated with organic matter decomposition, including guano deposits, basaltic caves, and marshlands. Similar conditions are found when human bladders are infected by urease-producing bacteria, when wastewater is treated, etc.

Struvite is occasionally found in canned seafood, where its appearance is that of small glass slivers, objectionable to consumers for aesthetic reasons but of no health consequence. A simple test can differentiate struvite from glass.

Struvite kidney stones are also known as triple phosphate stones (calcium magnesium ammonium phosphate), owing to the presence of carbonate apatite that precipitates to accompany struvite at high pH. Struvite and carbonate apatite precipitate in alkaline urine, forming kidney stones. Struvite is the most common mineral found in urinary tract stones in dogs, and is found also in urinary tract stones of cats and humans. Struvite stones are potentiated by bacterial infection that hydrolyzes urea to ammonium and raises urine pH to neutral or alkaline values. Urea-splitting organisms include Proteus, Xanthomonas, Pseudomonas, Klebsiella, Staphylococcus, and Mycoplasma.^{[citation needed]}

Even in the absence of infection, accumulation of struvite crystals in the urinary bladder is a problem frequently seen in housecats, with symptoms including difficulty urinating (which may be mistaken for constipation) or blood in the urine (hematuria). The protein cauxin, a protein excreted in large amounts in cat urine that acts to produce a feline pheromone, has recently been found to cause nucleation of struvite crystals in a model system containing the ions necessary to form struvite. This may explain some of the excess struvite production in domestic cats. In the past, surgery has been required to remove struvite uroliths in cats; today, special acidifying low magnesium diets may be used to dissolve sterile struvite stones.

Upper urinary tract stones that involve the renal pelvis and extend into at least two calyces are classified as staghorn calculi.^{[citation needed]} Although all types of urinary stones can potentially form staghorn calculi, approximately 75% are composed of a struvite-carbonate-apatite matrix.^{[citation needed]}