  The antiferromagnetic (AFM) semimetal YbMnSb$_2$ has recently been identified
as a candidate topological material, driven by time-reversal symmetry breaking.
Depending on the ordered arrangement of Mn spins below the N\'{e}el
temperature, $T_\mathrm{N}$ = 345 K, the electronic bands near the Fermi energy
can ether have a Dirac node, a Weyl node or a nodal line. We have investigated
the ground state magnetic structure of YbMnSb$_2$ using unpolarized and
polarized single crystal neutron diffraction. We find that the Mn moments lie
along the $c$ axis of the $P4/nmm$ space group and are arranged in a C-type AFM
structure, which implies the existence of gapped Dirac nodes near the Fermi
level. The results highlight how different magnetic structures can critically
affect the topological nature of fermions in semimetals.
