  The structure of the layered transition-metal Borides $A$B$_2$ ($A =$ Os, Ru)
is built up by alternating $T$ and B layers with the B layers forming a
puckered honeycomb. Here we report superconducting properties of RuB$_2$ with a
$T_c \approx 1.5$K using measurements of the magnetic susceptibility versus
temperature $T$, magnetization $M$ versus magnetic field $H$, resistivity
versus $T$, and heat capacity versus $T$ at various $H$. We observe a reduced
heat capacity anomaly at $T_c$ given by $\Delta C/\gamma T_c \approx 1.1$
suggesting multi-gap superconductivity. Strong support for this is obtained by
the successful fitting of the electronic specific heat data to a two-gap model
with gap values $\Delta_1/k_BT_c \approx 1.88$ and $\Delta_2/k_BT_c \approx
1.13$. Additionally, $M$ versus $H$ measurements reveal a behaviour consistent
with Type-I superconductivity. This is confirmed by estimates of the
Ginzburg-Landau parameter $\kappa \approx 0.1$--$0.66$. These results strongly
suggest multi-gap Type-I superconductivity in RuB$_2$. We also calculate the
band structure and obtain the Fermi surface for RuB$_2$. The Fermi surface
consists of one quasi-two-dimensional sheet and two nested ellipsoidal sheets
very similar to OsB$_2$. An additional small $4^{\rm th}$ sheet is also found
for RuB$_2$. RuB$_2$ could thus be a rare example of a multi-gap Type-I
superconductor.
