A tomato sandwich is a dish closely associated with the cuisine of the Southern United States. Recipes typically call for ripe-to-overripe non-commercially grown tomatoes, mayonnaise, salt, and pepper on soft commercial white bread. It is generally expected to be messy to eat.

According to Tasting Table, the Virginia Chronicle is "widely credited" with the first mention of the sandwich in 1911. According to Gwinnett Magazine, the Chronicle reported a man describing his lunch as "a tomato sandwich, a slice of watermelon, iced tea, and a slice of coconut cream pie".

The tomato sandwich is associated with Southern cuisine and according to Yahoo News is considered an important part of that cuisine. According to Chuck Reece, editor of Georgia Public Radio's Salvation South, the tomato sandwich is "one thing—one perfect thing—about which every Southerner can agree". The New York Times called it "the sandwich southerners wait for all year". Jenn Rice, writing in Garden & Gun, says "The taste of tomato slathered in mayo is such a part of our summer memories that it’s practically part of our DNA."

Outside of the south the tomato sandwich is not well-known and is sometimes mocked; Seattle food writer Geraldine DeRuiter caused a stir when she opined that "a 'tomato sandwich' is not a sandwich. You just don't have the ingredients to make a BLT". North Carolina YouTuber SouthernASMR posted a video of herself making and eating a tomato sandwich and was widely mocked by "plenty of ... non-southerners", some calling the sandwich "gross", according to the New York Times. Southern Living writer Rick Bragg said a common reaction from those outside the south when hearing of the sandwich was "yuck".

The tomato sandwich is mentioned in Harriet the Spy by Louise Fitzhugh. Harriet ate a tomato sandwich for lunch every day.

The Southern-style sandwich is made with soft white bread, mayonnaise, salt, and pepper. According to Southern Living, this "basic formula ... produces a Southern ideal" and a "true taste of summer in the South". According to The Southern Belle Primer, "If you feel like being creative, you might add a pinch of garlic, or, if you’re very daring, even a little curry powder."

The sandwich is ideally made with a fresh ripe-to-overripe tomato from a home garden or farmers' market; according to Southern Living, Serious Eats, and Bitter Southerner, a commercially grown supermarket tomato, which may have been picked unripe and stored refrigerated, "typically lacks the same level of juiciness, sweetness, and flavor complexity". Heirloom tomatoes such as Beefsteak are commonly recommended. A single slice of tomato the same size as the bread and at least as thick provides a sufficient filling while preventing thinner slices of tomato from slipping out of the sandwich.

The bread most commonly called-for is an untoasted soft commercial white sandwich bread such as Sunbeam, Sara Lee or Wonder Bread. Some recipes call for other soft white breads such as challah, brioche or Japanese milk bread, but Bill Smith, former chef at Crook's Corner, says "Use the cheapest store-brand white bread you can find ... Never, ever use any kind of fancy, artisanal loaf." Heirloom tomato expert John Coykendall, speaking to HuffPost, said "you have to have that old, cheap, white bread. The kind you wouldn’t ordinarily touch in your daily life. It’s the one thing that it was created for, tomato sandwiches". Some southern sandwich makers toast the bread, but more typically untoasted is specified. Mike Barnhardt of the Davie County Enterprise Record wrote "BLTs on toasted bread, yes; tomato sandwiches on toasted bread, no."