A heterojunction made by coexposed anatase (001)-(101) surfaces is studied using an explicit atomistic model of the interface via density functional theory. High photoactivity for this system has been demonstrated recently. Usually, the nature of a semiconductor heterojunction is evaluated by looking at band edges of the separate, noninteracting units, thus neglecting interfacial effects. Our results show non-negligible structural and electronic effects occurring at the junction, but because of the canceling nature of these effects, the alignment of the bands is qualitatively similar for the real interface and for the separated, noninteracting fragments. We also show from first principles that upon light absorption and electron excitation, the junction promotes charge carrier separation via localization of holes at O ions of the (001) side and electrons at Ti ions of the (101) side of the junction. This hinders recombination and is most likely the reason for high photoactivity.