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In this paper, we report a novel slow-wave coplanar waveguide (CPW) tri-section stepped-impedance resonator (SIR) that offers further size reduction compared to the conventional two-section SIRs. In this new design, the slow wave effect was realized by using the central section in a tri-section SIR to introduce effective capacitive coupling to the ground lines. The principle of using this slow-wave effect to achieve size reduction is described first and a circuit model is developed to explain it further. Simulation results confirm this kind of size reduction. Finally, to demonstrate the ability of the proposed SIRs in shrinking the size of filters, the tri-section slow-wave SIRs were implemented in a two-pole directly-coupled bandpass filter. It is shown that the size of the filter using our new tri-section SIR is about 70% of the one using the conventional two-section SIR.