Arkansas settled it by refusing to choose
Most states pick a fruit and a vegetable. Arkansas picked one thing for both jobs. Act 255 of 1987, signed by Governor Bill Clinton on March 16 of that year, designates the South Arkansas Vine Ripe Pink Tomato as the official state fruit and the official state vegetable — a single symbol occupying two slots, on the explicit reasoning that the tomato is botanically a fruit and used as a vegetable.
It is the most honest answer any legislature has given to this question, and it is worth keeping in mind before you vote: the state that grows the thing declined to take a side.
The Supreme Court did take one, for tax reasons
The federal answer is older and blunter. Under the Tariff Act of 1883, imported vegetables were taxed and imported fruit came in free. An importer named John Nix argued the botanical case and sued the collector of the Port of New York to get his money back. In Nix v. Hedden (1893) the Supreme Court ruled unanimously against him.
Justice Horace Gray conceded the botany in the first paragraphs, then set it aside: in the common language of the people, tomatoes are served at dinner with the soup, fish or meat, and not as dessert. The decision is a rare piece of American law that openly chooses everyday usage over scientific accuracy — and it did so because there was a tariff attached.
The science, briefly, because it is not in doubt
A fruit is the structure that develops from the ovary of a flower and carries the seeds. A tomato does exactly that, which makes it a berry, in the same technical family as the pepper, the cucumber, the eggplant and the pumpkin. There is no botanical counterargument, and nobody serious offers one.
What matters more is the other half: "vegetable" is not a scientific category at all. No botanist can define it, because it is a kitchen word describing how a plant is used. The two sides of this argument are therefore not contradicting each other. They are answering different questions and reporting the results as though they were the same answer.
So what are you actually voting on?
Vote on where the tomato belongs in your life, not on what you learned in ninth grade biology. A useful test: does it turn up in your fruit bowl, or on your cutting board next to the onion? If you would not put it in a fruit salad, you have your answer, and it does not make you wrong about botany.
One last thing worth knowing about the American position specifically. The tomato arrived here as a suspicious foreign plant, was widely believed to be poisonous well into the nineteenth century, and became a staple only after it had been thoroughly absorbed into savory cooking. The country learned to eat it as a vegetable before it ever had a reason to ask what it was. That head start still shows up in every vote.
Sources: Arkansas Act 255 of 1987 (Ark. Code § 1-4-115), state fruit and vegetable designation; Nix v. Hedden, 149 U.S. 304 (1893), opinion by Justice Horace Gray; Tariff Act of 1883; standard botanical definitions of fruit and berry.