  The vast majority of Galactic supernova remnants (SNRs) were detected by
their synchrotron radio emission. Recently, the evolved SNR G107.0+9.0 with a
diameter of about 3~deg or 75~pc up to 100~pc in size was optically detected
with an indication of faint associated radio emission. This SNR requires a
detailed radio study. We aim to search for radio emission from SNR G107.0+9.0
by analysing new data from the Effelsberg 100-m and the Urumqi 25-m radio
telescopes in addition to available radio surveys. Radio SNRs outside of the
Galactic plane, where confusion is rare, must be very faint if they have not
been identified so far. Guided by the H$\alpha$ emission of G107.0+9.0, we
separated its radio emission from the Galactic large-scale emission. Radio
emission from SNR G107.0+9.0 is detected between 22~MHz and 4.8~GHz with a
steep non-thermal spectrum, which confirms G107.0+9.0 as an SNR. Its surface
brightness is among the lowest known for Galactic SNRs. Polarised emission is
clearly detected at 1.4~GHz but is fainter at 4.8~GHz. We interpret the
polarised emission as being caused by a Faraday screen associated with
G107.0+9.0 and its surroundings. Its ordered magnetic field along the line of
sight is below 1~$\mu$G. At 4.8~GHz, we identified a depolarised filament along
the western periphery of G107.0+9.0 with a magnetic field strength along the
line of sight $B{_{||}} \sim 15~\mu$G, which requires magnetic field
compression. G107.0+9.0 adds to the currently small number of known, evolved,
large-diameter, low-surface-brightness Galactic SNRs. We have shown that such
objects can be successfully extracted from radio-continuum surveys despite the
dominating large-scale diffuse Galactic emission.
