This paper describes an active phase shifter, which exhibits a frequency-independent phase shift over the ultrawideband (UWB) frequency range. The design of this pure phase shifter relies on the use of a negative group delay (NGD) circuit. The topology of the active NGD cell is based on a Field Effect Transistor in cascade with a series resonant network. A 90°±5° phase shifter with a single NGD stage was designed, at first, and implemented in hybrid microstrip technology; its measurements showed a 75% relative bandwidth and validated the proposed approach. Then, a compact pure phase shifter with a multi-stage NGD cell was designed. It was aimed at meeting specifications in the UWB frequency range, i.e. 3.1-10.6 GHz (110% relative band). Over this band, the corresponding simulation results showed a constant transmission phase of -45°±3°, an insertion gain from 2 to 5.5 dB and insertion losses better than 9 dB.