We present the first results from the X-Shooter Lens Survey: an analysis of the massive early-type galaxy SDSS J1148+1930 at redshift z= 0.444. We combine its extended kinematic profile – derived from spectra obtained with X-Shooter on the European Southern Observatory Very Large Telescope – with strong gravitational lensing and multicolour information derived from Sloan Digital Sky Survey (SDSS) images. Our main results are as follows. (i) The luminosity-weighted stellar velocity dispersion is 〈σ*〉(≲Reff) = 352 ± 10 ± 16 km s−1, extracted from a rectangular aperture of 1.8 × 1.6 arcsec2 centred on the galaxy, more accurate and considerably lower than a previously published value of ∼450 km s−1. (ii) A single-component (stellar plus dark) mass model of the lens galaxy yields a logarithmic total-density slope of γ′= 1.72+0.05− 0.06 (68 per cent confidence level, CL; ) within a projected radius of ∼2.16 arcsec. (iii) The projected stellar mass fraction, derived solely from the lensing and dynamical data, is f*(<RE) = 0.19+0.04− 0.09 (68 per cent CL) inside the Einstein radius for a Hernquist stellar profile and no anisotropy. The dark matter fraction inside the effective radius fDM(<Reff) = 0.60+0.15− 0.06± 0.1 (68 per cent CL), where the latter error is systematic. (iv) Based on the SDSS colours, we find f*, Salp(<RE) = 0.17 ± 0.06 for a Salpeter initial mass function (IMF) and f*, Chab(<RE) = 0.07 ± 0.02 for a Chabrier IMF. The lensing and dynamics constraints on the stellar mass fraction agree well with those independently derived from the SDSS colours for a Salpeter IMF, which is preferred over a Chabrier IMF at variance with standard results for lower mass galaxies. Dwarf-rich IMFs in the lower mass range of 0.1–0.7 M⊙, with α≥ 3 (with dN/dM∝M−α) – such as that recently suggested for massive early-type galaxies with α= 3 in the mass range 0.1–1 M⊙– are excluded at the >90 per cent CL and in some cases violate the total lensing-derived mass limit. We conclude that this very massive early-type galaxy is dark-matter-dominated inside one effective radius, consistent with the trend recently found from massive Sloan Lens ACS (SLACS) galaxies, with a total density slope shallower than isothermal and an IMF normalization consistent with Salpeter.