  (Abridged) We present a detailed study of the host galaxy of GRB 011121 (at z
= 0.36) based on high-resolution imaging in 5 broad-band, optical and
near-infrared filters with HST and VLT/ISAAC. The surface brightness profile of
this galaxy is best fitted by a Sersic law with index ~ 2 - 2.5 and a rather
large effective radius (~ 7.5 kpc). Both the morphological analysis and the
F450W - F702W colour image suggest that the host galaxy of GRB 011121 is either
a disk-system with a rather small bulge, or one hosting a central,
dust-enshrouded starburst. Hence, we modeled the integrated spectral energy
distribution of this galaxy by combining stellar population and radiative
transfer models, assuming properties representative of nearby starburst or
normal star-forming, Sbc-like galaxies. A range of plausible fitting solutions
indicates that the host galaxy of GRB 011121 has a stellar mass of 3.1 - 6.9
x10^9 Msun, stellar populations with a maximum age ranging from 0.4 to 2 Gyr,
and a metallicity ranging from 1 to 29 per cent of the solar value. Starburst
models suggest this galaxy to be nearly as opaque as local starbursts (with an
A_V = 0.27 - 0.76 mag). Alternatively, normal star-forming Sbc-like models
suggest a high central opacity whereas A_V = 0.12$ -- 0.57 mag along the line
of sight. For this subluminous galaxy (with L_B/Lstar_B = 0.26), we determine a
model-dependent SFR of 2.4 - 9.4 Msun/yr. The SFR per unit luminosity (9.2 -
36.1 Msun/yr/(L_B/Lstar_B)) is high compared to those of most GRB host
galaxies, but consistent with those of most of the hosts at similar low
redshift. Our results suggest that the host galaxy of GRB 011121 is a rather
large disk-system in a relatively early phase of its star formation history.
