  We report on a newly detected point-like source, HESS J1943+213 located in
the Galactic plane. This source coincides with an unidentified hard X-ray
source IGR J19443+2117, which was proposed to have radio and infrared
counterparts. HESS J1943+213 is detected at the significance level of 7.9
\sigma (post-trials) at RA(J2000)=19h 43m 55s +- 1s (stat) +- 1s (sys),
DEC(J2000) = +21deg 18' 8" +- 17" (stat) +- 20" (sys). The source has a soft
spectrum with photon index Gamma = 3.1 +- 0.3 (stat) +- 0.2 (sys) and a flux
above 470 GeV of 1.3 +- 0.2 (stat) +- 0.3 (sys) x 10^{-12} cm^{-2} s^{-1}.
There is no Fermi/LAT counterpart down to a flux limit of 6 x 10^{-9} cm^{-2}
s^{-1} in the 0.1-100 GeV energy range (95% confidence upper limit calculated
for an assumed power-law model with a photon index Gamma=2.0). The data from
radio to VHE gamma-rays do not show any significant variability. We combine new
H.E.S.S., Fermi/LAT and Nancay Radio Telescope observations with pre-existing
non-simultaneous multi-wavelength observations of IGR J19443+2117 and discuss
the likely source associations as well as the interpretation as an active
galactic nucleus, a gamma-ray binary or a pulsar wind nebula. The lack of a
massive stellar counterpart disfavors the binary hypothesis, while the soft VHE
spectrum would be very unusual in case of a pulsar wind nebula. In addition,
the distance estimates for Galactic counterparts places them outside of the
Milky Way. All available observations favor an interpretation as an extreme,
high-frequency peaked BL Lac object with a redshift z>0.14. This would be the
first time a blazar is detected serendipitously from ground-based VHE
observations, and the first VHE AGN detected in the Galactic Plane.

High Energy Astrophysical Phenomena