HD 138965 is a young A type star and member of the nearby young Argus association. This star is surrounded by a broad, bright debris disc with two temperature components that was spatially resolved at far-infrared wavelengths by Herschel. Here, we present Atacama Large Millimiter/Submillimeter Array (ALMA) millimetre-wavelength imaging of the cool outer belt. These reveal its radial extent to be 150$^{+10}_{-7}$ au with a width ($\sigma$) of 49$^{+7}_{-6}$ au ($\Delta R/R = 0.77$), at a moderate inclination of 49${_{.}^{\circ}}$9$^{+3.3}_{-3.7}$. Due to the limited angular resolution, signal-to-noise and inclination we have no constraint on the disc's vertical scale height. We modelled the disc emission with both gravitational and radiation forces acting on the dust grains. As the inner belt has not been spatially resolved, we fixed its radius and width prior to modelling the outer belt. We find astronomical silicate is the best fit for the dust composition. However, we could not reject possible scenarios where there are at least 10 per cent water-ice inclusions. Combining the spatially resolved imaging by ALMA with non-detection at optical wavelengths by HST, we obtain a limit on the scattering albedo $\omega \le 0.09$ for the debris dust in the outer belt. Analysis of the outer belt's architecture in conjunction with simple stirring models places a mass limit of 2.3 $\pm$ 0.4 $M_{\rm Jup}$ on a companion interior to the belt ($a \le$ 78 au), a factor of two improvement over constraints from high contrast imaging.