A fishy smell from your penis is almost always caused by bacterial buildup, often from trapped sweat and skin cells under the foreskin or in skin folds. Less commonly, it can signal a sexually transmitted infection or, in rare cases, a metabolic condition. The good news: most causes are straightforward to fix.
Smegma and Bacterial Buildup
The most common explanation is smegma, a natural combination of oils, dead skin cells, and sweat that accumulates around the genitals. In uncircumcised men, the foreskin traps these substances and creates a warm, moist environment where bacteria thrive. As those bacteria feed on the buildup, they produce a strong odor that can smell sour, musty, or fishy. Smegma itself isn’t harmful, but letting it accumulate makes the smell worse over time.
Even circumcised men can develop odor from bacteria colonizing the skin folds around the groin, especially after sweating. The groin stays warm and enclosed for most of the day, which is exactly what odor-producing bacteria prefer.
How to Clean Properly
If you’re uncircumcised, gently pull back the foreskin during every shower and wash underneath with warm water. A mild, unscented soap works fine. The key is consistency: skipping even a couple of days lets smegma rebuild. After washing, dry the area thoroughly before pulling the foreskin back into place. Moisture left behind just restarts the cycle.
Your underwear matters more than you might think. Cotton is breathable and wicks away sweat, which limits bacterial growth. Synthetic fabrics trap moisture against the skin. Even underwear marketed with a cotton crotch panel doesn’t fully protect you, since the surrounding synthetic fabric still restricts airflow. Changing underwear daily (or twice daily if you sweat heavily) makes a noticeable difference. Sleeping without underwear gives the area a chance to air out overnight.
Trichomoniasis and STIs
If improved hygiene doesn’t resolve the smell within a week or two, a sexually transmitted infection called trichomoniasis is worth considering. It’s caused by a parasite and is more common than most people realize: the CDC estimated over two million infections in the United States in 2018 alone. In women, trichomoniasis is well known for producing a fishy-smelling discharge, and men can pick up the infection and develop symptoms of their own.
Men with trichomoniasis may notice itching or irritation inside the penis, burning after urinating or ejaculating, and unusual discharge. Many men have no symptoms at all, which means you can carry and transmit the infection without knowing it. Testing is simple: a urine sample is usually enough for a diagnosis. Treatment for men is typically a single oral dose of an antibiotic, and the infection clears quickly once treated. Sexual partners need treatment at the same time to prevent reinfection.
Other STIs like chlamydia and gonorrhea can also cause discharge with an unpleasant smell, though the odor is less characteristically “fishy.” These are diagnosed through urine tests or swabs from the urethra.
Urinary Tract Infections
UTIs are less common in men than in women, but they do happen. When urine becomes concentrated with waste products, it produces a strong ammonia-like smell that can linger on the skin of the penis after urination. If you’re also experiencing burning during urination, frequent urges to pee, or cloudy urine, a UTI could be the source. A simple urine test confirms the diagnosis.
Trimethylaminuria: A Rare Metabolic Cause
If the fishy smell persists despite good hygiene and negative infection tests, a rare genetic condition called trimethylaminuria (TMAU) could be responsible. People with this condition lack a functioning version of a liver enzyme that normally breaks down trimethylamine, the exact compound that gives fish their smell. Without that enzyme working properly, trimethylamine builds up and gets released through sweat, urine, breath, and reproductive fluids.
TMAU is present from birth, so if you’ve dealt with a persistent fishy body odor for most of your life, it’s worth mentioning to a doctor. Managing it involves dietary changes (reducing foods high in the compound’s precursors, like certain fish, eggs, and beans), minimizing heavy sweating, and using slightly acidic soaps that help neutralize the trimethylamine on your skin before it becomes volatile enough to smell.
Signs That Need Urgent Attention
A fishy smell on its own is rarely dangerous, but certain combinations of symptoms point to something serious. Fournier’s gangrene is a rare, life-threatening infection of the genital skin that produces a foul or putrid smell alongside rapidly worsening symptoms: skin that turns red, purple, or dark, significant swelling, sudden pain ranging from mild to severe, and fever of 100.4°F or higher. This condition progresses in hours, not days, and is fatal in nearly 30% of cases. If you notice discolored or swollen genital skin along with fever and feeling generally unwell, go to the emergency room immediately.
Narrowing Down the Cause
Start with the simplest explanation. Wash thoroughly for a week, switch to cotton underwear, and see if the smell resolves. If it doesn’t, or if you notice discharge, burning, itching, or pain, get tested. STI screening involves a urine sample or a swab and results come back quickly. Most causes of genital odor in men are either a hygiene issue that resolves with better routine or an infection that clears with a short course of treatment.