We report the detection of three transiting planets around a Sunlike star, which we designate Kepler-18. The transit signals were detected in photometric data from the Kepler satellite, and were confirmed to arise from planets using a combination of large transit-timing variations, radial-velocity variations, Warm-Spitzer observations, and statistical analysis of false-positive probabilities. The Kepler-18 star has a mass of 0.97M_sun, radius 1.1R_sun, effective temperature 5345K, and iron abundance [Fe/H]= +0.19. The planets have orbital periods of approximately 3.5, 7.6 and 14.9 days. The innermost planet b is a super-Earth with mass 6.9 pm 3.4M_earth, radius 2.00 pm 0.10R_earth, and mean density 4.9 pm 2.4 g cm^-3. The two outer planets c and d are both low-density Neptune-mass planets. Kepler-18c has a mass of 17.3 pm 1.9M_earth, radius 5.49 pm 0.26R_earth, and mean density 0.59 pm 0.07 g cm^-3, while Kepler-18d has a mass of 16.4 pm 1.4M_earth, radius 6.98 pm 0.33R_earth, and mean density 0.27 pm 0.03 g cm^-3. Kepler-18c and Kepler-18d have orbital periods near a 2:1 mean-motion resonance, leading to large and readily detected transit timing variations.
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