  We present the first detailed study on the Galactic supernova remnant (SNR)
G46.8-0.3 and its environment since its discovery more than 50 years ago. With
new flux measurements from radio surveys plus a compilation from the literature
we created an improved integrated radio spectrum of G46.8-0.3, which is well
fitted by a simple power-law with an index alpha = -0.535 +/- 0.012, then
excluding the presence of absorption by abundant ionised gas in the line of
sight, or in the SNR's proximity. The analysis of local changes in the radio
spectral index across G46.8-0.3 suggests a slight steepening at about 1 GHz,
which does not impact on the integrated spectrum of the source. From atomic
hydrogen (HI) spectra we placed the remnant at 8.7 +/- 1.0 kpc, and revisited
the distance to the nearby HII region G046.495-00.241 to 7.3 +/- 1.2 kpc. From
evolutionary models and our distance estimate, we conclude G46.8-0.3 is a
middle-aged ($\sim$1 x 10^4 yr) SNR. We recognised several 12CO and 13CO
molecular structures in the proximity of the remnant and used combined CO-HI
profiles to derive the kinematic distances to these features and characterise
their physical properties. We also provided compelling evidences for
environmental molecular clouds physically linked to G46.8-0.3 at its centre, on
its eastern edge, and towards the northern and southwestern rims on the far
side of the SNR shell. Our study does not confirm that the remnant is embedded
in a molecular cavity as previously suggested. G46.8-0.3 shows a line-of-sight
coincidence with the gamma-ray source 4FGL J1918.1+1215c, detected at GeV
energies by the space telescope Fermi. A rough analysis based on the properties
of the interstellar matter close G46.8-0.3 indicates that the GeV gamma-rays
photons detected in direction to the SNR can be plausibly attributed to
hadronic collisions and/or bremsstrahlung radiation.
