Trigonitis is the presence of metaplasia from urothelium to nonkeratinizing squamous epithelium at the trigone of the bladder. The trigone is the area of the bladder that is between the ureteral openings and the internal urethral sphincter. 

Trigonitis is usually asymptomatic. It can sometimes present with urinary frequency, urgency, and pain. Unlike keratinizing squamous metaplasia, it is not a premalignant lesion. While its name suggests inflammation, it is more accurately described as a result of chronic irritation.

Infection and indwelling catheters may predispose a patient to trigonitis. It most often occurs in premenopausal women and affects almost 40% of adult females and less than 5% of adult males.

Nonkeratinizing squamous metaplasia may occur in other locations in the urinary tract.