We report the discovery of HAT-P-27b, an exoplanet transiting the moderately bright G8 dwarf star GSC 0333-00351 (V = 12.214). The orbital period is 3.039586 ± 0.000012 days, the reference epoch of transit is 2455186.01879 ± 0.00054 (BJD), and the transit duration is 0.0705 ± 0.0019 days. The host star with its effective temperature 5300 ± 90 K is somewhat cooler than the Sun and is more metal-rich with a metallicity of +0.29 ± 0.10. Its mass is 0.94 ± 0.04 M⊙ and radius is 0.90+0.05 - 0.04 R⊙. For the planetary companion we determine a mass of 0.660 ± 0.033 M J and radius of 1.038+0.077 - 0.058 R J. For the 30 known transiting exoplanets between 0.3 M J and 0.8 M J, a negative correlation between host star metallicity and planetary radius and an additional dependence of planetary radius on equilibrium temperature are confirmed at a high level of statistical significance.