  Pressure evolution of the crystal structure and magnetism of the honeycomb
$\alpha$-RuBr$_3$ is studied using high-pressure x-ray diffraction,
magnetometry, and density-functional band-structure calculations. Hydrostatic
compression transforms antiferromagnetic $\alpha$-RuBr$_3$ ($R\bar 3$) into
paramagnetic $\alpha'$-RuBr$_3$ ($P\bar 1$) where short Ru-Ru bonds cause
magnetism collapse above 1.3 GPa at 0 K and 2.5 GPa at 295 K. Below this
critical pressure, the N\'eel temperature of $\alpha$-RuBr$_3$ increases with
the slope of 1.8 K/GPa. Pressure tunes $\alpha$-RuBr$_3$ away from the Kitaev
limit, whereas increased third-neighbor in-plane coupling and interlayer
coupling lead to a further stabilization of the collinear zigzag state. Both
$\alpha$- and $\alpha'$-RuBr$_3$ are metastable at ambient pressure, but their
transformation into the thermodynamically stable $\beta$-polymorph is
kinetically hindered at room temperature.
