  We discovered superconductivity at $T_c$ = 12 K in a layered compound
La$_2$IOs$_2$ with osmium honeycomb network. Despite heavy constituent elements
unfavorable for phonon mediated mechanism, $T_c$ is the highest among
lanthanoid iodides made of lighter elements such as La$_2$IRu$_2$ with $T_c$ =
4.8 K. Electronic anomalies are observed below 60 K similar to those observed
in La$_2$IRu$_2$ below 140 K. La$_2$IOs$_2$ is a layered 5d electron system
providing a platform to investigate the interplay between the electronic
anomaly, superconductivity, and strong magnetic field.
