[1][2][3] Lygaeus kalmii was first formally named in 1874 by Swedish entomologist Carl Stål in his Enumeratio Hemipterorum.

[4][3] There are two recognized subspecies:[3] Young nymphs of Lygaeus kalmii are initially fully red, developing black diagonal markings on the pronotum.

The forewings are patterned with a black heart inside of an orange-red X which does not meet in the middle.

[2][5] The membranous portion of the forewings in the eastern subspecies Lygaeus kalmii angustomarginatus has a narrow white margin, while that of the western L. k. kalmii has white spots and a broader margin.

[2] The small milkweed bug is commonly confused with other black and red or orange insects, including Oncopeltus fasciatus, Lygaeus turcicus, and Lygaeus reclivatus.