  Rare-earth diantimondes exhibit coupling between structural and electronic
orders which are tunable under pressure and temperature. Here we present the
discovery of a new polymorph of LaSb$_2$ stabilized in thin films synthesized
using molecular beam epitaxy. Using diffraction, electron microscopy, and first
principles calculations we identify a YbSb$_2$-type monoclinic lattice as a
yet-uncharacterized stacking configuration. The material hosts
superconductivity with a $T_\mathrm{c}$ = 2 K, which is enhanced relative to
the bulk ambient phase, and a long superconducting coherence length of 140 nm.
This result highlights the potential thin film growth has in stabilizing novel
stacking configurations in quasi-two dimensional compounds with competing
layered structures.
