In earlier papers the dipole moment of Mercury was estimated to be approximately 5¿1022 G cm3 by a spherical harmonic analysis of Mariner 10 encounter data obtained in March 1974 and March 1975 in which the external field was represented by harmonics of order 1 or 2 and the internal field was represented by a centered, tilted dipole. This paper presents an alternative model for the magnetospheric magnetic field of Mercury in which the external field is represented by an image dipole and a tail field and the internal field includes a dipole, a quadrupole, and an octupole. The dipole moment estimated by this model is approximately 2.4¿1022 G cm3, tilted 2.3¿ from the normal to the planetary orbital plane and having the same directional sense as that of the earth. The dipole, quadrupole, and octupole moment intensities are in the approximate ratios 1:0.4:03, respectively. All planetary field lines of the model magnetosphere are confined to a magnetospherelike region. Results are obtained which show the geometry, field line configuration, and field isointensity contours inside the magnetosphere of Mercury.