We report the discovery of HAT-P-26b , a transiting extrasolar planet orbiting the moderately bright V=11.744 K1 dwarf star GSC 0320-01027 , with a period P = 4.234516 \pm 0.000015 d , transit epoch T _ { c } = 2455304.65122 \pm 0.00035 ( BJD Barycentric Julian dates throughout the paper are calculated from Coordinated Universal Time ( UTC ) ) , and transit duration 0.1023 \pm 0.0010 d. The host star has a mass of 0.82 \pm 0.03 M _ { \sun } , radius of 0.79 _ { -0.04 } ^ { +0.10 } R _ { \sun } , effective temperature 5079 \pm 88 K , and metallicity [ Fe / H ] = -0.04 \pm 0.08 . The planetary companion has a mass of 0.059 \pm 0.007 M _ { J } , and radius of 0.565 _ { -0.032 } ^ { +0.072 } R _ { J } yielding a mean density of 0.40 \pm 0.10 g cm ^ { -3 } . HAT-P-26b is the fourth Neptune-mass transiting planet discovered to date . It has a mass that is comparable to those of Neptune and Uranus , and slightly smaller than those of the other transiting Super-Neptunes , but a radius that is \sim 65 % larger than those of Neptune and Uranus , and also larger than those of the other transiting Super-Neptunes . HAT-P-26b is consistent with theoretical models of an irradiated Neptune-mass planet with a 10 M _ { \earth } heavy element core that comprises \gtrsim 50 \% of its mass with the remainder contained in a significant hydrogen-helium envelope , though the exact composition is uncertain as there are significant differences between various theoretical models at the Neptune-mass regime . The equatorial declination of the star makes it easily accessible to both Northern and Southern ground-based facilities for follow-up observations .