  We report the discovery of the brightest X-ray source hosted by a faint
(M_B=-16) dwarf galaxy in the immediate vicinity of the ultraluminous IRAS
merging galaxy Mrk 273. The dwarf galaxy, 1.3 arcmin away from Mrk 273, is at
the tip of a faint northeast plume of Mrk 273. Its spectrum exhibits strong
[OIII], Halpha, [NII] emission lines, which establish the redshift of the dwarf
galaxy, $z=0.0376$, the same as that of Mrk 273. The emission line ratios are
typical of Seyfert galaxies. The X-ray emission is consistent with a point-like
source coincident with the center of the dwarf galaxy. The intrinsic X-ray
luminosity, 6.3x10^{41} ergs, in the 0.1--2.4 keV energy range, is about seven
times larger than the B band luminosity. The X-ray spectrum of the source can
be fit with a power-law. All the evidence is consistent with the source being a
Seyfert galaxy. It is mysterious why out of $\sim 10$ faint objects in the same
field only one is detected by ROSAT and its ratio of soft X-ray to optical
luminosity is as high as those for BL Lac objects and few active galactic
nuclei (AGNs). If there is a population of such dwarf AGNs hidden as companions
of major merger galaxies (such as Mrk 273), they may contribute to the
luminosity function of AGNs and the cosmic X-ray background at the faint end.
