  Very high-energy gamma-ray emission from PKS 0447-439 was detected with the
H.E.S.S. Cherenkov telescope array in December 2009. This blazar is one of the
brightest extragalactic objects in the Fermi Bright Source List and has a hard
spectrum in the MeV to GeV range. In the TeV range, a photon index of 3.89 +-
0.37 (stat) +- 0.22 (sys) and a flux normalisation at 1 TeV, Phi(1 TeV) = (3.5
+- 1.1 (stat) +- 0.9 (sys)) x 10^{-13} cm^{-2} s^{-1} TeV^{-1}, were found. The
detection with H.E.S.S. triggered observations in the X-ray band with the Swift
and RXTE telescopes. Simultaneous UV and optical data from Swift UVOT and data
from the optical telescopes ATOM and ROTSE are also available. The spectrum and
light curve measured with H.E.S.S. are presented and compared to the
multi-wavelength data at lower energies. A rapid flare is seen in the Swift XRT
and RXTE data, together with a flux variation in the UV band, at a time scale
of the order of one day. A firm upper limit of z < 0.59 on the redshift of PKS
0447-439 is derived from the combined Fermi-LAT and H.E.S.S. data, given the
assumptions that there is no upturn in the intrinsic spectrum above the
Fermi-LAT energy range and that absorption on the Extragalactic Background
Light (EBL) is not weaker than the lower limit provided by current models. The
spectral energy distribution is well described by a simple one-zone Synchrotron
Self-Compton (SSC) scenario, if the redshift of the source is less than z <~
0.4.
