  We study BPS domain walls in four-dimensional $\mathcal{N}=1$ massive SQCD
with gauge group $SU(N)$ and $F<N$ flavors. We propose a class of
three-dimensional Chern-Simons-matter theories to describe the effective
dynamics on the walls. Our proposal passes several checks, including the exact
matching between its vacua and the solutions to the four-dimensional BPS domain
wall equations, that we solve in the small mass regime. As the flavor mass is
varied, domain walls undergo a second-order phase transition, where multiple
vacua coalesce into a single one. For special values of the parameters, the
phase transition exhibits supersymmetry enhancement. Our proposal includes and
extends previous results in the literature, providing a complete picture of BPS
domain walls for $F<N$ massive SQCD. A similar picture holds also for SQCD with
gauge group $Sp(N)$ and $F < N+1$ flavors.
