  We report on the results of powder and single crystal neutron diffraction to
investigate the magnetic order in YbCo$_2$Si$_2$ below the N\'eel temperature
$T_{\rm N} = 1.7~$K in detail. Two different magnetically ordered phases can
clearly be distinguished. At lowest temperatures a commensurate magnetic
structure with a propagation vector ${\bf k}_1 = (0.25~ 0.25~ 1)$ is found,
while the intermediate phase ($T > 0.9~$K) is characterized by an
incommensurate magnetic structure with ${\bf k}_2 = (0.25~ 0.086~ 1)$. The
magnetic structure in YbCo$_2$Si$_2$ is in marked contrast to all other known
RCo$_2$Si$_2$ compounds (R = rare earth element) likely due to some itineracy
of the Yb 4f states being responsible for the magnetism.
