The short answer: Tomatoes and tomatillos are relatives, not twins. Both are nightshades, but tomatoes are Solanum lycopersicum and tomatillos are Physalis philadelphica. Tomatillos have a papery husk, firmer flesh and a brighter tart flavor. They are not green tomatoes wearing little jackets.
Tomatillos Are Not Unripe Tomatoes
This is the part that saves dinner.
A tomatillo is its own plant in the Physalis genus. A tomato belongs to Solanum. They share the nightshade family, which makes them botanical cousins rather than the same produce item at different stages of emotional development.
Utah State University Extension describes tomatillos as related to tomatoes but distinct, with their characteristic papery husks and tart fruit.
The Husk Is the Obvious Giveaway
Tomatillos grow inside a thin papery husk that looks like somebody wrapped each fruit individually because produce apparently needed gift packaging.
Remove the husk and you will usually find a slightly sticky surface. Rinse that off before cooking.
Tomatoes Are Juicier; Tomatillos Are Firmer and Tart
Tomatoes generally bring more juice, sweetness and softness. Tomatillos are firmer and notably tangier, with the sharp flavor that makes salsa verde taste like salsa verde rather than green-colored tomato sauce.
Roasting tomatillos mellows some of that sharpness and adds deeper flavor. Raw tomatillos keep more of their bright acidic edge.
Can You Substitute Tomatoes for Tomatillos?
You can substitute almost anything for anything if nobody is allowed to stop you.
But they are not a true one-for-one swap when the flavor matters. Utah State Extension specifically notes that other fruits may substitute in some uses, but not if you want the authentic tomatillo flavor.
If a recipe is built around tomatillos—especially salsa verde—use tomatillos.
Where Each One Works Best
Tomatoes are everywhere for a reason: salads, sandwiches, soups, pasta sauces, pizza and the annual August problem of suddenly owning seventeen tomatoes.
Tomatillos are especially important in Mexican cooking, including salsa verde, sauces, stews and roasted preparations where their tartness is the point rather than an inconvenience to be negotiated.
Tomatoes vs. Tomatillos in One Sentence
Tomatoes are juicy and familiar; tomatillos are husked, firmer and tart.
Same extended family. Different personality. Do not send one to work wearing the other’s name tag.