  We report the magnetic and electric properties of Ba$_3$NiNb$_2$O$_9$, which
is a quasi-two-dimensional spin-1 triangular lattice antiferromagnet (TLAF)
with trigonal structure. At low $T$ and with increasing magnetic field, the
system evolves from a 120 degree magnetic ordering phase (A phase) to an
up-up-down ($uud$) phase (B phase) with a change of slope at 1/3 of the
saturation magnetization, and then to an "oblique" phase (C phase).
Accordingly, the ferroelectricity switches on at each phase boundary with
appearance of spontaneous polarization. Therefore, Ba$_3$NiNb$_2$O$_9$ is a
unique TLAF exhibiting both $uud$ phase and multiferroicity.
