Radio galaxy phenomenon is directly connected to mass accreting, spinning supermassive black holes found in the active galactic nuclei (AGN). It is still unclear how the collimated jets of relativistic plasma on hundreds to thousands of kpc scale form, and why nearly always they are launched from the nuclei of bulge dominated elliptical galaxies and not flat spirals. Here we present the discovery of giant radio source J2345-0449 (z=0.0755), a clear and extremely rare counter example where relativistic jets are ejected from a luminous and massive spiral galaxy on scale of ~1.6 Mpc, the largest known so far. Extreme physical properties observed for this bulgeless spiral host, such as its high optical and infra-red luminosity, large dynamical mass, rapid disk rotation, and episodic jet activity are possibly the results of its unusual formation history, which has also assembled, via gas accretion from a disk, its central black hole of mass >2 x 10^8 M_sun. The very high mid-IR luminosity of the galaxy suggests that it is actively forming stars and still building a massive disk. We argue that the launch of these powerful jets is facilitated by an advection dominated, magnetized accretion flow at low Eddington rate onto this unusually massive (for a bulgeless disk galaxy) and possibly fast-spinning central black hole. Therefore, J2345-0449 is an extremely rare, unusual galactic system whose properties challenge the standard paradigms for black hole growth and formation of relativistic jets in disk galaxies. Thus, it provides fundamental insight into accretion disk -- relativistic jet coupling processes.
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