The physical and chemical properties of the circumgalactic medium (CGM) at z ≳ 6 have been studied successfully through the absorption in the spectra of background Quasi-Stellar Objects (QSOs) 1–3. One of the most crucial questions is to investigate the nature and location of the source galaxies that give rise to these early metal absorbers4–6. Theoretical models suggest that momentum-driven outflows from typical star-forming galaxies can eject metals into the CGM and the intergalactic medium (IGM), with the projected separation between absorbers and galaxies expected to range from 5 to 50 proper kpc at z=5–6 7–10. Deep, dedicated surveys have searched for Lyα emission associated with strong CIV absorbers at z ≈ 6, but only a few Lyα emitter candidates have been detected. Interpreting these detections is moreover ambiguous because Lyα is a resonant line11–13, raising the need for complementary techniques for detecting absorbers’ host galaxies. Here, using Atacama Large Millimeter Array (ALMA), we report, for the first time, the [C II] 158µm emission and the far-infrared dust continuum associated with a strong low-ionization absorber, O I, at z = 5.978. The [C II] luminosity is 73 million solar luminosities, corresponding to a dark matter halo mass of 100 billion solar masses. This is one to two orders of magnitude more massive than typical values predicted from cosmological simulations9, 16.