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We present the first VLBI synthesis map of the luminous H$_2$O maser in the nucleus of NGC 4258, for epoch 1984.77. The maser spectral features were distributed on an East--West arc in the sky, subtending about 250 microarcseconds ($\mu$as) in angle ($2.6\times10^{16}$ cm). The distribution was not resolved in the North-South direction ($\ll 500 \mu$as). The Doppler velocity of the maser emission changes with position along the arc with an essentially constant gradient of $0.0391\pm0.0002$ km s$^{-1}$ AU$^{-1}$. The most redshifted emission lie farthest to the East for these observations. This source is at least an order of magnitude more compact than the other extremely luminous extragalactic H$_2$O maser that has been mapped with VLBI techniques (i.e., NGC 3079). If the angular distribution of the maser emission traces an underlying gravitationally bound rotating disk then the observed range of velocities and angular extent imply an enclosed mass of $\sim 700$ M$_\odot$.
\noindent The extraordinary luminosity of the maser is not consistent with what is seen in regions of star formation within our galaxy. We suggest that the H$_2$O maser emission from the nucleus NGC 4258 may originate in a circumnuclear molecular ``ring,'' stimulated by a background, high brightness temperature radio continuum source known to exist in the nucleus or by H$_2$O maser emission from the back side of the circumnuclear ring. The VLBI observations reported here provide important clues and constraints on the proposed model. High velocity H$_2$O maser emission ($\pm800$ km s$^{-1}$) has recently been discovered in this galaxy. VLBI observations (Nakai et al. ) show that these are spatially coincident with the low velocity components to within about 2 pc.
