This letter proposes a novel design method of the single-layer ultrawideband (UWB) 90° phase shifter, which consists of a multimode resonator with shunt stubs and longitudinally cascaded coupled lines. The physical insight and working mechanism of its ultrawide operation bandwidth are extensively illustrated in the view of group delay for the main and reference lines. Meanwhile, the return loss and phase deviation can be appropriately adjusted in the design process. The measured results exhibit its bandwidth is 122% of < ±5.4°, return loss of >10 dB, and insertion loss of <1.04 dB. The comparison shows the proposed design not only achieves wide bandwidth but also attain small phase deviation, simple geometry, and compact size. The predicted results have been well validated in the experiment.