  Let $\Phi$, $\Phi'$ be Leonard systems over a field $\mathbb{K}$, and $V$,
$V'$ the vector spaces underlying $\Phi$, $\Phi'$, respectively. In this paper,
we introduce and discuss a balanced bilinear form on $V\times V'$. Such a form
naturally arises in the study of $Q$-polynomial distance-regular graphs. We
characterize a balanced bilinear form from several points of view.
