  By means of symmetry analysis, density functional theory calculations, and
Monte Carlo simulations we show that goethite, $\alpha$-FeOOH, is a linear
magnetoelectric below its N\'{e}el temperature $T_{\rm N}=400$ K. The
experimentally observed magnetic field induced spin-flop phase transition
results in either change of direction of electric polarization or its
suppression. Calculated value of magnetoelectric coefficient is 0.24 $\mu$C
m$^{-2}$ T$^{-1}$. The abundance of goethite in nature makes it arguably the
most widespread magnetoelectric material.
