We have found a new low-mass, double-lined, detached eclipsing binary, GU Boo, among a sample of new variables from the ROTSE-I database. The binary has an orbital period of 0.488728±0.000002 days and estimated apparent magnitudes Vrotse≃13.7 and I≃11.8. Our analysis of the light and radial velocity curves of the system yields individual masses and radii of M1=0.610±0.007 M☉, M2=0.599±0.006 M☉, R1=0.623±0.016 R☉, and R2=0.620±0.020 R☉. The stars in GU Boo are therefore very similar to the components of the eclipsing binary YY Gem. For this study we have adopted a mean effective temperature for the binary of Teff=3870±130 K. Based on its space velocities we suggest that GU Boo is a main-sequence binary, possibly with an age of several Gyr. The metallicity of the binary is not well constrained at this point, but we speculate that it should not be very different from solar. We have compared the physical parameters of GU Boo with current low-mass stellar models, accounting for uncertainties in age and metallicity by considering a wide range of values for those parameters. Our comparisons reveal that all the models underestimate the radii of the components of GU Boo by at least 10%-15%. This result is in agreement with the recent studies of YY Gem and CU Cnc.