We report the first weak lensing detection of a large-scale filament funnelling matter on to the core of the massive galaxy cluster MACS J0717.5+3745. Our analysis is based on a mosaic of 18 multipassband images obtained with the Advanced Camera for Surveys aboard the Hubble Space Telescope, covering an area of ˜10 × 20 arcmin2. We use a weak lensing pipeline developed for the Cosmic Evolution Survey, modified for the analysis of galaxy clusters, to produce a weak lensing catalogue. A mass map is then computed by applying a weak gravitational lensing multiscale reconstruction technique designed to describe irregular mass distributions such as the one investigated here. We test the resulting mass map by comparing the mass distribution inferred for the cluster core with the one derived from strong lensing constraints and find excellent agreement. Our analysis detects the MACS J0717.5+3745 filament within the 3σ detection contour of the lensing mass reconstruction, and underlines the importance of filaments for theoretical and numerical models of the mass distribution in the cosmic web. We measure the filament's projected length as ˜4.5 h74-1 Mpc, and its mean density as (2.92 ± 0.66) × 108 h74 M&sun; kpc-2. Combined with the redshift distribution of galaxies obtained after an extensive spectroscopic follow-up in the area, we can rule out any projection effect resulting from the chance alignment on the sky of unrelated galaxy group-scale structures. Assuming plausible constraints concerning the structure's geometry based on its galaxy velocity field, we construct a three-dimensional (3D) model of the large-scale filament. Within this framework, we derive the 3D length of the filament to be 18 h74-1 Mpc. The filament's deprojected density in terms of the critical density of the Universe is measured as (206 ± 46) ρcrit, a value that lies at the very high end of the range predicted by numerical simulations. Finally, we study the distribution of stellar mass in the field of MACS J0717.5+3749 and, adopting a mean mass-to-light ratio of 0.73 ± 0.22 and assuming a Chabrier initial mass function, measure a stellar mass fraction along the filament of (0.9 ± 0.2) per cent, consistent with previous measurements in the vicinity of massive clusters.