  Aims. With an observed and rest-frame duration of < 2s and < 0.5s,
respectively, GRB090426 could be classified as a short GRB. The prompt
detection, both from space and ground-based telescopes, of a bright optical
counterpart to this GRB offered a unique opportunity to complete a detailed
study. Methods. Based on an extensive ground-based observational campaign, we
obtained the spectrum of the optical afterglow of GRB090426, measuring its
redshift and obtaining information about the medium in which the event took
place. We completed follow-up observation of the afterglow optical light curve
down to the brightness level of the host galaxy that we firmly identified and
studied. We also retrieved and analyzed all the available high-energy data of
this event, and compared the results with our findings in the optical. This
represents one of the most detailed studies of a short-duration event presented
so far. Results. The time properties qualify GRB090426 as a short burst. In
this case, its redshift of z = 2.61 would be the highest yet found for a GRB of
this class. On the other hand, the spectral and energy properties are more
similar to those of long bursts. LBT late-time deep imaging identifies a
star-forming galaxy at a redshift consistent with that of the GRB. The
afterglow lies within the light of its host and shows evidence of local
absorption.
