Lactarius indigo, commonly known as the indigo milk cap, indigo milky, indigo lactarius, blue lactarius, or blue milk mushroom, is a species of agaric fungus in the family Russulaceae.

The fruit body color ranges from dark blue in fresh specimens to pale blue-gray in older ones. The "milk", or latex, that oozes when the mushroom tissue is cut or broken (a feature common to all members of the genus Lactarius) is also indigo blue, but slowly turns green upon exposure to air. The cap has a diameter of 4–15 cm (2–6 in), and the stem is 2–8 cm (3⁄4–3+1⁄8 in) tall and 1–2.5 cm (3⁄8–1 in) thick.

It is a widely distributed species, growing in the Americas and Eurasia. L. indigo grows on the ground in both deciduous and coniferous forests, where it forms mycorrhizal associations with a broad range of trees. It is an edible mushroom and is sold in rural markets in Mexico, Guatemala, and China.