A planar broadband metamaterial absorber with high absorptivity working at terahertz frequencies was designed and fabricated in this work. Two nested back-to-back split-ring resonators (BSRRs) constitute a single resonator, which achieves three strong resonances, with two of them merged into a broadband peak. Cobalt silicide and parylene-C were innovatively applied as ground plane and dielectric spacer. The nested BSRR absorber experimentally realizes a bandwidth of 0.66 THz with the absorptivity above 0.8, and the highest absorptivity reaches 0.97. Taking the central frequency at 2.74 THz, the measured FWHM is 47% and the Q factor is 2.13.