  Bulk rutile RuO$_2$ has long been considered a Pauli paramagnet. Here we
report that RuO$_2$ exhibits a hitherto undetected lattice distortion below
approximately 900 K. The distortion is accompanied by antiferromagnetic order
up to at least 300 K with a small room temperature magnetic moment of
approximately 0.05 $\mu_B$ as evidenced by polarized neutron diffraction.
Density functional theory plus $U$ (DFT+$U$) calculations indicate that
antiferromagnetism is favored even for small values of the Hubbard $U$ of the
order of 1 eV. The antiferromagnetism may be traced to a Fermi surface
instability, lifting the band degeneracy imposed by the rutile crystal field.
The combination of high N\'eel temperature and small itinerant moments make
RuO$_2$ unique among ruthenate compounds and among oxide materials in general.
