I'd like to preface this with saying I've been a bike commuter for just about one year now and still know very little about bicycle maintenance, just the basics. Anyways, I live in Buffalo, NY and kept my bike away for the winter and it stayed in very good condition.

Last weekend I rode it to my grandpa's birthday party through the rain. I was so exhausted that I left it in their shed and just got back to pick it up (the weather's finally perfect biking weather). Anyways, I started riding home and looked down and noticed my chain has developed quite a bit of rust in its 9 day stay in their shed (which I guess didn't surprise me that much because everything in their shed is rusty...).

So what's the best way to remedy this? Surely I don't need to get a new one yet, right? I'm really not certain about what to do because like I said, I'm pretty unknowledgeable when it comes to bike maintenance and I'd rather admit defeat and get help than really mess it up.

Just oil it and rub it down well with a coarse rag. The oil and rubbing will remove any loose particles of ferric oxide and the oil will darken the surace rust and neutralize any further damage.

There's rust removal options but it won't reverse the surface damage. And if your chain is a black oxide finish sort of chain the dark "rust" spots left after the oil and wipe cleaning won't be obvious at all.

The fact that it rusted suggests that whatever chain oil you're using is less than ideal.

Thanks! I use Finish Line Teflon-Plus Dry lube..

A good lubricant like Pro Link will take off superficial rust, or you could use solvent. Wiope it off with a clean rag and re-lube. If the chain is really rusty, that is, if Pro link doesn't get it off, change it.

The best way to prevent rust on the chain is a fender and mud flap. Well, not eaxactly, the best is a fully enclosed chain case, but that would only work if you have an internally geared hub or a single speed.

If you ride in the rain wipe off the chain and lube it right after you finish riding.

The fact that you DIDN`T post that the chain was seized up is a good indication that the lube inside the chain hadn`t been washed out completely. Any corrosion on the outside plates of the chain is pretty superficial, but you want to keep the rollers and internals corrosion free.

Two reasons for that: (1) Iron oxide takes up mose space than iron and has a rougher texture. That causes parts to sieze and increases friction on the ones that don`t. (2) Most metal oxides are actually harder than the pure metal and iron is no exception. In fact iron oxide is as hard as a hardened steel file.

So the combination of those will accelerate wear in bothe whole drive system. A chain can be inexpensive, but you don`t want to have to start replacing chainrings and cassettes any sonner than you have to.

WD-40 is a good thing to keep around if you drive in the rain a lot. It displaces water and is easy to apply . As a lubricant its too light weight to last any amount of time, but it can buy you some time and let you do a proper lube when you have the chance.

If you're going to make it a habit to ride in wet conditions then a dry lube is not going to work. Dry lubes with teflon powder work OK as a lubricant but they do nothing for oxidation protection. For that you need good ol' oil. It takes very little oil to provide a rust protective film but it does require that you use SOME oil, grease or other film style lubricant that resists being washed away by water.

4 parts unscented mineral spirits to 1 part chain saw bar oil from ACE hardware. Remove and clean the chain at least every 500 miles.
https://www.sheldonbrown.com/brandt/chain-care.html

A chain will start to show rust if left in the right conditions & wet overnight, put some oil on it, like Finish Line WET lube, ride it for a few miles, and the rust will be gone; now if the chain had been left in that state for a few months, a new chain would be in order

+1000 I agree 1,000% here. You wouldn't get deep enough rust to matter in only a week or so, so oil it decently, and wipe off excess oil and any rust that comes with and you'll be good to go. The key to rust is to prevent it because there's no cure. Keep your chain decently lubed (whatever works best for you) and weather protected, and don't sweat a few spots now and then.

I save my rusty chains in my junk box.

If the chain still turns without any stiff links, oil it and keep riding.

Cosmetic rust is nothing more than unsightly as long as the chain still tests within length limits. Like others have said, just oil it and go. It's not going to hurt anything. It's not the end of the world.

Use the ruler test to make sure the chain isn't out of spec and check to see if your cogs/chainwheels are not too worn. A good investment is a Park Tool chain gauge which takes the guesswork out of checking a chain and doesn't require crawling around on your knees or putting the bike up on a stand to quickly check your chain's condition. It's a quick go/no-go slap-on gauge that takes 10 seconds to use.

Running a worn-out "stretched" chain will damage your cogs and chainwheels in a hurry so you want to keep an eye on that. Rust on the outside of the chain isn't going to hurt anything though.

In the end chains are cheap disposable wear items that you'll go through rather quickly if you ride a lot. Many people go nuts with fancy nostrum snake oils and lubricaion crap and spend way more money trying to get an extra tiny bit of life out of what is a $10-15 disposable item (unless of course you are riding a recumbent or tandem with 152 feet of chain on it.) Throw some oil on it every once in a while and let it go until it's time to replace it with a new one because of "stretch" wear. If you buy a chain online it's not any bigger of a deal than replacing a tube or a tyre.

Any old oil will do that doesn't make too much of a mess.

What you probably have is surface rust. That means the chain probably just looks bad but very little internal damage has been done and can be cleaned off. But make sure you wipe down all the sprockets on the BB and freewheel. Rust is an excellent grinding material and needs to be kept off all mechanical parts. And yes if you ride anywhere or time the chain will get wet, use some form of oil rather than dry lub.