  We report the magnetic properties of $A$-site spinel compound MnSc$_2$Se$_4$.
The macroscopic magnetic measurements uncovers successive magnetic transitions
at $T_{\rm{N1}}$= 2.04 K, followed by two further transitions at
$T_{\rm{N2}}$=1.8 K and $T_{\rm{N3}}$=1.6 K. Neutron powder diffraction reveals
that both, $T_{\rm{N2}} < T < T_{\rm{N1}}$ and $T <T_{\rm{N3}}$, orders are
associated with the propagation vector $k$=(3/4 3/4 0), while the magnetic
structures are collinear amplitude modulated and helical, respectively. Using
neutron powder spectroscopy we demonstrated the effect of substitution of S by
Se on the magnetic exchange. The energy range of the spin-wave excitations is
supressed due to the chemical pressure of the $X$- ion in MnSc$_2X_4$ ($X$=S,
Se) spinels.
