Trichomoniasis (trich) is an infectious disease caused by the parasite Trichomonas vaginalis. About 70% of affected people do not have symptoms when infected. When symptoms occur, they typically begin 5 to 28 days after exposure. Symptoms can include itching in the genital area, a bad smelling thin vaginal discharge, burning with urination, and pain with sex. Having trichomoniasis increases the risk of getting HIV/AIDS. It may also cause complications during pregnancy.

Trichomoniasis is a sexually transmitted infection (STI) which is most often spread through vaginal, oral, or anal sex. It can also spread through genital touching. People who are infected may spread the disease even when symptoms are not present. Diagnosis is by finding the parasite in the vaginal fluid using a microscope, culturing the vaginal fluid or urine, or testing for the parasite's DNA. If present, other STIs should be tested for.

Methods of prevention include not having sex, using condoms, not douching, and being tested for STIs before having sex with a new partner. Although not caused by a bacteria, trichomoniasis can be cured with certain antibiotics (metronidazole, tinidazole, secnidazole). Sexual partners should also be treated. About 20% of people get infected again within three months of treatment.

There were about 122 million new cases of trichomoniasis in 2015. In the United States, there are about 2 million women affected. It occurs more often in women than men. Trichomonas vaginalis was first identified in 1836 by Alfred Donné. It was first recognized as causing this disease in 1916.