A metamaterial balun that converts a single-ended input to a differential output over a large bandwidth is presented. The device also exhibits excellent return loss, isolation, and through characteristics over the same frequency band. The balun comprises a Wilkinson divider, followed by a +90° negative-refractive-index (NRI) metamaterial (MM) phase-shifting line along the top branch, and a -90° MM phase-shifting line along the bottom branch. Utilizing MM lines for both the +90° and -90° branches allows the slopes of their phase responses to be matched, resulting in a broadband differential output signal. The theoretical performance of the balun is verified through circuit simulations and measurements of a fabricated prototype at 1.5 GHz. The MM balun exhibits a measured differential output phase bandwidth (180° ± 10°) of 1.16 GHz (77%), from 1.17 to 2.33 GHz. The measured isolation and return loss for all three ports remain below -10 dB over a bandwidth in excess of 2 GHz, while the output quantities |S 21| and |S31| remain above -4 dB from 0.5 to 2.5 GHz.