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We present observations of a large-scale disc of neutral hydrogen (H I) in the nearby Fanaroff–Riley type I (FR I) radio galaxy NGC 3801 with the Westerbork Synthesis Radio Telescope. The H I disc (34 kpc in diameter and with M_Hi = 1.3 x 10⁹M_☉) is aligned with the radio jet axis. This makes NGC 3801 an ideal system for investigating the evolution of a small radio source through its host galaxy's cold interstellar medium (ISM). The large-scale H I disc is perpendicular to a known inner CO disc and dust lane. We argue that the formation history of the large-scale H I disc is in agreement with earlier speculation that NGC 3801 was involved in a past gas-rich galaxy–galaxy merger (although other formation histories are discussed). The fact that NGC 3801 is located in an environment of several H I-rich companions, and shows indications of ongoing interaction with the nearby companion NGC 3802, strengthens this possibility. The large amounts of ambient cold ISM, combined with X-ray results by Croston, Kraft & Hardcastle on the presence of overpressured radio jets and evidence for an obscuring torus, are properties that are generally not, or no longer, associated with more evolved FR I radio sources. We do show, however, that the H I properties of NGC 3801 are comparable to those of a significant fraction of nearby low-power compact radio sources, suggesting that studies of NGC 3801 may reveal important insight into a more general phase in the evolution of at least a significant fraction of nearby radio galaxies.