This paper presents a miniaturized 3-dB branch-line coupler based on slow-wave microstrip transmission lines. The miniaturized coupler operating at 2.45 GHz is designed and implemented on a double-layer PCB substrate with blind metallic vias embedded in the lower substrate layer providing the slow-wave effect. Based on this concept, a 43% size miniaturization is achieved as compared to classical microstrip branch-line coupler prototype. The measured S parameters present a return loss of 20 dB and an insertion loss of 3.36 dB at the design frequency.