Cornichons are French-style pickles, and they may not be as readily available at your local grocery store than other types of pickles. If you run into the problem of not being able to find cornichons, there are plenty of cornichon substitutes available.
The closest substitutes for cornichons are dill pickles, dill gherkins, salted cucumbers, zucchini, chutney, white vinegar, and other pickled vegetables. The substitute you choose will depend on the recipe’s use of cornichon.

What Can I Substitute for Cornichons?
With many pickled vegetables on the market, there are a variety of substitutes you can use instead of cornichons. Some substitutes will vary in taste and texture, so choose your substitute according to your intended use of cornichon.
Dill Pickles
Dill pickles are the closest substitute to cornichons. They have the same flavor, but the texture will be slightly different. You can find dill pickles in multiple shapes and sizes, depending on your needs.
Dill Gherkins
Dill gherkins are very similar to dill pickles and cornichons, but they are slightly different. Gherkins and cucumber are in the same family of vegetable, but gherkins are not quite as crunchy as cornichons are. In a pinch, dill gherkins will give you the same flavor as cornichons.
Salted Cucumbers
Salted cucumbers are a homemade option to cornichons. To achieve a similar flavor to cornichons, soak long slices of cucumbers in salt, tarragon, and vinegar.

Zucchini
Zucchinis are in the same gourd family as cucumbers and gherkins and can be treated in the same way as cucumbers (above). Salted zucchini will be denser and not as moist as cucumber or cornichons, but you will be able to achieve the same taste and a similar texture.
Chutney
Chutney is very similar in taste to cornichons, displaying the same tanginess, sourness, and sweetness that cornichons do. However, chutney is a condiment and not a vegetable, so it can only be used in certain recipes. Chutney has substitutes as well.
White Vinegar
If you only need a sharpness and zest in your dish from the cornichon, white vinegar will add just that. The texture will be different from cornichon, so keep that in mind when choosing a substitute.
Pickled Vegetables
A variety of vegetables can be pickled, and they will all have a similar texture and taste to cornichons. Pickled carrots, cabbage, and radish are all good substitutes to cornichons.