From optical photometry we show that SDSS J121258.25-012310.1 is a new eclipsing, post common-envelope binary with an orbital period of 8.06 h and an eclipse length of 23 min. We observed the object over 11 nights in different bands and determined the ephemeris of the eclipse to HJDmid = 2 454 104.7086(2) + 0.3358706(5) × E, where numbers in parenthesis indicate the uncertainties in the last digit. The depth of the eclipse is 2.85 ± 0.17 mag in the V band, 1.82 ± 0.08 mag in the R band and 0.52 ± 0.02 mag in the I band. From spectroscopic observations we measured the semi-amplitude of the radial velocity K2 = 181 ± 3 km s-1 for the secondary star. The stellar and binary parameters of the system were constrained from a) fitting the SDSS composite spectrum of the binary, b) using a K-band luminosty-mass relation for the secondary star, and c) from detailed analyses of the eclipse light curve. The white dwarf has an effective temperature of 17 700 ± 300 K, and its surface gravity is log g=7.53 ± 0.2. We estimate that the spectral type of the red dwarf is M4 ± 1 and the distance to the system is 230 ± 20 parsec. The mass of the secondary star is estimated to be in the range M_sec= 0.26-0.29 M_&sun; , while the mass of the white dwarf is most likely M_wd= 0.46-0.48 M_&sun;. From an empirical mass-radius relation we estimate the radius of the red dwarf to be in the range 0.28-0.31 R_&sun; , whereas we get R_wd= 0.016-0.018 R_&sun; from a theoretical mass-radius realation. Finally we discuss the spectral energy distribution and the likely evolutionary state of SDSS1212-0123. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.