Stagnant water, that is the water in your rain barrel, can over time develop a putrid odor. That would be from the algae that builds up in stagnant water. Empty the barrel and flush it really good and when it fills up again use the water out of it sooner then you have been. You might want to consider putting an aerator in the barrel to reintroduce some air into the water.
Depending too on what was the original purpose of the barrel that could be contributing some to the odor.

Effective microorganisms.

Here is a link that might be useful: EM

Put a 3-inch layer of compost in the bottom and aerate.

You're not drinking the water are you?

You might also try using a quarter of a Mosquito Dunk. Those things do more than just kill mosquitoes.

Thanks for the suggestions. Washing the rain barrel is sort of counter productive since I'm using it to save water. In that respect, it's working really well -- just about the time the barrel is nearly empty, we get more rain & it fills up within an hour. And I'm not sure about the compost idea since the spigot is so close to the bottom, I think the compost would block the flow. Also the aeration would involve electricity, which is something I try to use sparingly. (Plus I'd have to buy the aeration equipment.)

I'll look into the microorganism link. But in the meantime, I'll try a piece of the mosquito dunk. And if that doesn't work, I'll just go back to holding my breath!

You will hold your breath for reasons pennies will solve.
That would be a cap full of EM allso good for anyplace you would use that water.

Add a 1/4 cup of clorox to the barrel. It should fix any odor problems, and it will be so diluted that it won't be harmful to your plants.

A putrid odor emitting from any substance could indicate anaerobic processes, processes done in the absence of air. When the bacteria that are anaerobic are present often they are also disease pathogens. Putrid odors are never a good thing and indicate potential problems.

If your gutters have leaves or other organic materials, they are washed into the rain barrel and they are anaerobically decomposing in there at the bottom ... stinking up the barrel.

Dump it, wash it, and in the future, clean it out more frequently. Some way of preventing leaves and bugs from getting in the barrel would be good.

I found this thing amazing.
Just put ceramics in the bottom and no problem for many years.

Here is a link that might be useful: EM Ceramics for water purification.

I too have found Ginny the barrel smells. I also agree to wash it out is defeating the purpose. For me, I have my barrel up on cinder blocks, 4 In fact. This keeps the nosel away from the ground. It also allows the water to come out at a better speed. I emptied my today( using the water) and left the nosel open. I have just tilted for the sake of draining it... With it left open it allow air to flow from the top where the screen is out.
Hope it helps

I keep my rain barrels in pairs at each downspout. Mine sometimes smell bad as well. When one does, I just use it all in one barrel more frequently, then switch to the other, using it all. It just comes from organics in the water, algea growth, etc. Doesn't hurt anything.

Your rain barrel never gets empty? It only takes a few minutes to swish it out, rinse with a hose, maybe scrub the bottom with a corn broom to loosen the gunky coating. I will bet there is a good layer of sediment and leaves down in there.

I'm with tox, clean it out. Dump the water in the garden so as not to waste it. Don't let gunk build up at the bottom and go anaerobic.

I have the same problem. Does the stinky water hurt the plants? Is it good for them?

The stinky water: pour it on the soil and not on the plants (i.e. leaves or elsewhere). The aerobic bacteria in the soil should dispense with the anaerobic pretty quickly. For excessive caution, avoid food plants and especially those with leaves you eat (i.e. don't pour on lettuce).

I can't imagine what would make a rain barrel stink unless like someone said something fell in and drowned. I have seen that happen esp when there is a drought some critter gets thirsty then falls in. I would have to look inside to see if something did fall in.

I have several barrels both open and closed. The closed ones never seem to get algae but the open ones will. Mosquitoes don't bother with the closed ones either.

The sediment and leaves at the bottom of mine stinks when I clean it out because it's been underwater just like pond sediment. But the water doesn't normally smell. My barrels are essentially closed up though, with just a downspout inlet, so the entire surface of the water isn't exposed to the air. I don't notice an odor when I drain clear water out through a hose to water plants. But boy does that sediment stink when I hose it out.

Hydroponic gardeners keep water in reservoirs for days or weeks without it going foul by using an aerator (think aquarium air stone). The bad smell is from anaerobic bacteria and can make you sick. The aerator promotes aerobic bacteria which are also highly beneficial. I'd not consider the water potable and would keep it in the root zone.