A kitchen sink that smells like sewage is usually caused by a dry P-trap, the curved pipe under the sink that holds water to block sewer gas from rising back up the drain. When that water evaporates, often because the sink or a nearby drain sat unused, the seal breaks and the smell comes through. Refilling it is often a five-minute fix. A smell that returns after refilling, or that seems tied to venting, calls for a licensed plumber.
This sits alongside the full kitchen deep-clean order on House Tweaks and near cleaning a dishwasher filter, another spot where trapped debris causes a smell that gets blamed on the wrong thing.
Which smell is this: food odor from the disposal, or sewer gas?
Before assuming the worst, it's worth pinning down what kind of smell you're actually dealing with. Ordinary food odor, the kind that comes from a garbage disposal with buildup in its splash guard or grind chamber, tends to smell sour or rotten and is usually strongest right at the disposal opening. It builds up gradually, and the fix there is cleaning the disposal itself, not chasing a trap or a vent that was never the problem.
Sewer gas smells different: often described as a rotten-egg or distinctly sewage-like odor, and it's frequently noticeable across the whole kitchen rather than concentrated right at one fixture. If you've already ruled out the disposal and the smell still reads as sewage rather than food, the most likely explanation is the plumbing behind the sink rather than anything sitting in the sink itself.
The most common cause: a dry P-trap
The P-trap is the U-shaped or S-shaped bend of pipe you can usually see under the sink, and it's designed to hold a small amount of water at all times. That standing water acts as a seal, physically blocking sewer gas in the pipe below from traveling up through the drain and into your kitchen.
Every plumbing source that covers this topic agrees on the same thing: a dry trap is the single most common cause of a sewage smell at a sink, and it's also the easiest one to fix yourself. The water in the trap evaporates when a drain goes unused for a stretch, which is common with a rarely used bar sink, a secondary kitchen sink, or any drain that just doesn't see regular water flow. Once the seal is gone, the smell has a direct path into the room.
Refilling the trap, and running water down drains you rarely use
If a dry trap is the cause, the fix is straightforward: run water down the drain until you're confident the trap has refilled and the seal is restored. There's no need to overthink the process. Do this for the smelly drain first, then think about any other drain in the kitchen (or elsewhere in the house) that doesn't get used often, since the same evaporation can happen in any trap that sits idle.
If you know a particular drain, an unused bar sink, a floor drain, a rarely used laundry tub, goes long stretches without water, running it periodically is a simple way to keep this from recurring at all.
Grease and food buildup as a slower-building cause
A dry trap isn't the only explanation. Grease and food particles that go down the drain over time can coat the inside of the pipe and slowly decompose, along with biofilm that builds up on pipe walls in general. This tends to produce a smell that builds up gradually rather than appearing suddenly, and it's a different mechanism from a trap that's simply lost its water seal.
Keeping grease out of the drain in the first place, wiping pans before washing them and disposing of grease in the trash rather than pouring it down the sink, is the most effective way to prevent this cause specifically. If you do use a cleaning product to address existing buildup, follow its label exactly for how it's meant to be used, and never combine bleach with ammonia or with vinegar under any circumstances; that combination produces toxic gas, and a kitchen with several cleaning products under the sink is exactly where that mistake tends to happen by accident.
When it is not the trap: venting problems
If you've refilled the trap and the smell comes back, or if it never seemed tied to a specific drain sitting unused, the cause may be a venting problem rather than the trap itself. Plumbing vent stacks are what let air into the drain system so water can flow properly and gases can escape safely out through the roof instead of into your home. When a vent is blocked, disconnected, or improperly installed, sewer gas can find its way back into the house even with a properly sealed trap.
This is where the fix stops being something a homeowner should attempt. Vent stacks run behind walls and through the roof, diagnosing and correcting a venting issue is a licensed plumber's job, not a DIY project, and this piece isn't going to walk through pipe sizing or code specifics that vary by house and by jurisdiction anyway.
What a licensed plumber checks that a homeowner cannot
A licensed plumber has access to the parts of your drain and vent system a homeowner generally doesn't: they can run a camera down the line to see blockages or breaks you can't observe from under the sink, check the vent stack itself for obstructions or disconnections, and assess whether the trap or vent configuration for your specific plumbing setup is even correct in the first place. Older homes in particular sometimes have plumbing that was modified over the years in ways that created a venting problem nobody noticed until a smell like this showed up.
If refilling the trap doesn't resolve the smell, if it returns within a few days, or if you simply suspect something more than a dry trap, call a licensed plumber rather than continuing to guess. If the smell is persistent and strong in the meantime, ventilate the room and step back from the area rather than trying to assess whether it's something to worry about; that's a call for a professional, not a DIY diagnosis.
This site is upfront that home repairs aren't all DIY-safe just because a blog post explains them; more on that approach, and where the line is drawn, is on the about page.
FAQ
Why does my kitchen sink suddenly smell like sewage when it didn't before?
The most common reason is a P-trap that's lost its water seal, often because a drain went unused for a while and the water in the trap evaporated. Running water down the drain to refill it resolves this in most cases.
How do I know if it's the disposal or the drain that smells?
Food odor from a garbage disposal tends to smell sour and stays concentrated right at the disposal opening, while a sewage smell reads differently and is often noticeable across the kitchen rather than at one specific spot.
I refilled the trap and the smell is still there. What now?
That points away from a simple dry trap and toward either grease buildup further down the line or a venting problem. A venting issue isn't a DIY fix, so this is the point to call a licensed plumber.
Can I just use a strong cleaner to get rid of the smell?
A cleaner may mask the smell temporarily or help with grease buildup specifically, but it won't fix a dry trap or a venting problem, which are mechanical issues rather than a cleanliness issue. Follow any product's label exactly and never mix bleach with ammonia or vinegar.
Is a sewage smell in my kitchen dangerous?
This is a question for a professional rather than something to self-diagnose. If the smell is strong or persistent, ventilate the space and call a licensed plumber rather than trying to assess the risk yourself.
Do other drains in my house need attention too if one smells like sewage?
It's worth running water down any drain in your home that doesn't get used often, since the same evaporation that causes a dry trap in one sink can happen in any infrequently used fixture.
