Broadband THz metamaterial absorbers have important applications in terahertz modulation, imaging, and communication, but current designs usually involve complex nesting or stacking resonant structures. This paper reports a single-resonant-structure and optically transparent broadband terahertz metamaterial absorber. The absorber demonstrates > 80% broadband absorption in a wide frequency range of 0.4~1.04 THz, or 88.9% of the central frequency. The thickness of the absorber is only 1/15 of the wavelength corresponding to the lowest absorption frequency. The absorption spectrum has been experimentally verified with a reflective terahertz time-domain spectrometer. Further study shows that the absorber is insensitive to the polarization angle, and the absorbance decreases very little for incident angles less than 30°. The absorption spectrum can be finely tuned by varying the structure parameters.