We report the discovery of two new transiting extrasolar planets. HAT-P-18b orbits the V= 12.759 K2 dwarf star GSC 2594− 00646, with a period P= 5.508023±0.000006 days, transit epoch T c= 2454715.02174±0.00020 (BJD), and transit duration 0.1131±0.0009 days. The host star has a mass of 0.77±0.03 M☉, radius of 0.75±0.04 R☉, effective temperature 4803±80 K, and metallicity [Fe/H]=+ 0.10±0.08. The planetary companion has a mass of 0.197±0.013 M J and radius of 0.995±0.052 R J, yielding a mean density of 0.25±0.04 g cm− 3. HAT-P-19b orbits the V= 12.901 K1 dwarf star GSC 2283− 00589, with a period P= 4.008778±0.000006 days, transit epoch T c= 2455091.53417±0.00034 (BJD), and transit duration 0.1182±0.0014 days. The host star has a mass of 0.84±0.04 M☉, radius of 0.82±0.05 R☉, effective temperature 4990±130 K, and metallicity [Fe/H]=+ 0.23±0.08. The planetary companion has a mass of 0.292±0.018 M J and radius of 1.132±0.072 R J, yielding a mean density of 0.25±0.04 g cm− 3. The radial velocity residuals for HAT-P-19 exhibit a linear trend in time, which indicates the presence of a third body in the system. Comparing these observations with theoretical models, we find that HAT-P-18b and HAT-P-19b are each consistent with a hydrogen–helium-dominated gas giant planet with negligible core mass. HAT-P-18b and HAT-P-19b join HAT-P-12b and WASP-21b in an emerging group of low-density Saturn-mass planets, with negligible inferred core masses. However, unlike HAT-P-12b and WASP-21b, both HAT-P-18b and HAT-P-19b orbit stars with super-solar metallicity. This calls into question the heretofore suggestive correlation between the inferred core mass and host star metallicity for Saturn-mass planets.