  The magnetic, thermal and transport properties as well as electronic band
structure of MnPtSi are reported. MnPtSi is a metal that undergoes a
ferromagnetic transition at $T_{\mathrm{C}}=340$(1) K and a spin-reorientation
transition at $T_{\mathrm{N}}=326$(1) K to an antiferromagnetic phase.
First-principles electronic structure calculations indicate a not-fully
polarized spin state of Mn in a $d^5$ electron configuration with $J=S=3$/2, in
agreement with the saturation magnetization of 3~$\mu_{\mathrm{B}}$ in the
ordered state and the observed paramagnetic effective moment. A sizeable
anomalous Hall effect in the antiferromagnetic phase alongside the
computational study suggests that the antiferromagnetic structure is
non-collinear. Based on thermodynamic and resistivity data we construct a
magnetic phase diagram. Magnetization curves $M$($H$) at low temperatures
reveal a metamagnetic transition of spin-flop type. The spin-flopped phase
terminates at a critical point with $T_{\mathrm{cr}}\approx 300$ K and
$H_{\mathrm{cr}}\approx 10$ kOe, near which a peak of the magnetocaloric
entropy change is observed. Using Arrott plot analysis and magnetoresistivity
data we argue that the metamagnetic transition is of a first-order type,
whereas the strong field dependence of $T_{\mathrm{N}}$ and the linear
relationship of the $T_{\mathrm{N}}$ with $M^2$ hint at its magnetoelastic
nature.
