We show that the Smith normal form of a skew‐symmetric D‐optimal design of order
n
≡
20.3em
(
mod0.3em
4
) is determined by its order. Furthermore, we show that the Smith normal form of such a design can be written explicitly in terms of the order
n, thereby proving a recent conjecture of Armario. We apply our result to show that certain
D‐optimal designs of order
n
≡
20.3em
(
mod0.3em
4
) are not equivalent to any skew‐symmetric
D‐optimal design. We also provide a correction to a result in the literature on the Smith normal form of
D‐optimal designs.