A novel direct synthesis method for wideband linear-to-circular polarizers on 3-D frequency selective structures is presented. The proposed linear-to-circular polarizer consists of two orthogonal linearly-polarized transmission paths, both of which are comprised by three cascaded sections of slotline structures. Under the incidence of a linearly-polarized (LP) wave with its electric-field (E-field) vector of a titled angle 45°, each path can achieve a transmission passband for one of the two orthogonal components decomposed by the incident E-field vector. To obtain a transmitted wave with circularly-polarized (CP) property, the well-known conditions on magnitude and phase difference of these two paths are presented. Then, to develop a unified synthesis method, two new conditions of phase difference and phase slope at the center frequency are introduced to replace the typical phase condition. The closed-form equations are derived, and a synthesis procedure is given. It can be obtained that all the involved circuit parameters can be derived from the specifications of these two paths, such as in-band insertion-loss (IL) ripples and fractional bandwidths. Moreover, the phase difference condition of these two paths is further modified to obtain an arbitrary axial ratio (AR) at the center frequency and enhance the AR bandwidth of the polarizer. A group of design examples are theoretically investigated to demonstrate and verify the design concept. An alternative design example is finally selected and fabricated. Good agreement between simulated and measured results features its advantages of wide AR bandwidth and low IL. Therefore, the proposed synthesis method can provide a promising solution to design many other types of wideband linear-to-circular polarizers.