In this article, a 360° continuously tunable phase shifter (PS) is designed based on the proposed PS theory extended from the general tunable filter theory. The PS is composed of the reconfigurable multiple resonances and two external coupling networks (ECNs). The phase performances of the reconfigurable resonances are toggled among three states with different mode numbers to achieve consecutive 90° phase jumps. Dynamic and complex matching with positive imaginary parts is achieved between the reconfigurable resonances and ECNs, so as to obtain the smooth switch between phase-jumping states and the continuous fine phase shifting. In addition to the structural construction, a set of analytical formulas and guidelines for synthesizing the 270° PS is provided. Further, by loading two extra varactors on the microstrip line, the discrete phase jumps between the three states increase from 90° to 120° and the phase slope is more flexible, resulting in a complete 360° phase shift range (PSR) with low phase error. Finally, a tunable PS with reconfigurable three resonances is designed, fabricated, and measured. The proposed PS boasts a reduced total circuit size of 0.025°, a fractional bandwidth (FBW) of 15%, and a continuous PSR of 360°.