  We analyze 3.5 years of public Fermi/LAT data around the position of the
supernova remnant HB 21, where four point-like sources from the 2nd Fermi/LAT
catalog are located. We determine that the gamma-ray source is produced by a
single extended source. We model the observed morphology as a uniform circle.
The spectral energy distribution is best described by a curved power law, with
a maximum at 413+-11 MeV. We divide the circle into three regions defined by
previously identified shocked molecular clouds, and find that one of these
regions has a softer spectrum. The >3 GeV gamma-ray emission of the soft
spectrum region is bow-shaped and coincident with the supernova remnant shell
seen at radio wavelengths. We suggest that the gamma-ray emission from HB 21
can be understood as a combination of emission from shocked/illuminated
molecular clouds, one of them coincident with the supernova remnant shell
itself.
