Looks like a ground hog to me. I mean really it could be anything. But I have had out and out wars with them last few years. Not so much this year- maybe they got my message. I've seen them climb my indeterminate tomato trellis and be well off the ground. This tomato I gave one more day on the vine, I nearly cried!

I've read it's often because they are thirsty. Btw I'm sorry it's really frustrating!

Or rats. Or squirrels.

All depends on who lives in your part of the world.

Are squirrels a common tomato pest? I have never heard anyone who grows tomatoes talk about squirrels eating their tomatoes, but I think that is happening to me. Am I alone in this???

No, you are not alone. Lots of people have squirrel problems. Whether or not squirrels in your neighborhood eat your tomatoes has a lot to do with what else is available to them. I also think that individual squirrels have individual tastes, just like any other animal. So it's possible to just have squirrels around that like tomatoes whereas someone elsewhere may have squirrels that don't really care for them.

Definitely some kind of rodent. I had the same problem, turned out to be a squirrel in my case. I'm still figuring out the best way to deal with it. Here's a pic of the damage to my tomatoes

So... what's the best way to kill squirrels? I know maybe dozen people who grow tomatoes regularly and not one have ever mentioned squirrels, so I am flabbergasted, but erecting huge cages around my nicely-growing tomato plants is going to make harvesting the tomatoes a pain. I'm thinking poison, but how to ensure only the squirrels eat it???

Some people report luck with motion-detecting repellents that squirt water at animals that come near, etc. You could try bagging your tomatoes with something--nylons, etc.--that may make the fruit less attractive. Others have tried capsaicin (hot pepper) sprays. Or a dog. You'll likely have to try a lot of different things before finding a solution that works, as it seems all squirrel populations have their own quirks.

If you want to kill them, the best bet so you don't hurt neighbors' pets or other wildlife is to trap them, then kill them.

Probably woodchucks. They are a plague here, which is why I now have a 6 ft chain link fence around the veggie garden. Even so, I found last year that baby rabbits were able to squeeze through the fencing, and gobbled up half my kale and bean plants in a very short time, so I surrounded it with chicken wire and now its rodent free. I don't think there are too many repellents you can use that are safe for food, especially if its a woodchuck. I've found very few repellents that deter them. Your best bet is a fence. Of course, if its a squirrel or chipmunks, fencing won't keep them out either. My sister uses garlic powder and red pepper, she buys the super size containers and sprinkles it all over the plants. But, she does have 2 large dogs too, so I can't say if its the garlic/pepper or the dogs that keep them away.