We analyze an 850 mum Submillimetre Common-User Bolometer Array (SCUBA) map of the environment of the z = 2.39 radio galaxy 53W002, which has been shown to reside in an overdensity of Lyalpha-detected galaxies. We identify four luminous submillimeter sources within a 2.'3 (1.2 Mpc at z = 2.39) diameter area around the radio galaxy (which itself is a weak submillimeter source). We employ a 1.4 GHz map to accurately locate the counterpart of one of these sources, SMMJ17142+5016, and identify this source with a narrow-line active galactic nucleus with an extended Lyalpha halo at z = 2.390 that is a member of the structure around 53W002. Hence, SMM J17142+5016 is the first spectroscopically confirmed submillimeter-selected companion to a high-redshift radio galaxy. We discuss the OH-Airglow Suppression Spectrograph JHK spectrum of this galaxy and in addition present five new constraints on its spectral energy distribution longward of 1 mum, using these to estimate its bolometric luminosity as similar to8 x 10(12) L., or a star formation rate of similar to10(3) M. yr(-1) if young stars provide the bulk of the luminosity. This result provides direct support for the statistical detection of overdensities of SCUBA galaxies around high-redshift radio galaxies and confirms theoretical predictions that SCUBA galaxies, as the progenitors of massive elliptical galaxies, should be strongly clustered in the highest density regions of the distant universe.