A planar Marchand balun with double-layer coupled lines on an FR4 substrate is proposed. The double-layer coupled-line structure mitigates the low-coupling issue associated with the substrate’s high dissipation factor, enhances the coupling coefficient of the coupled lines, and consequently improves the balun performance. An isolation circuit is employed to provide isolation and impedance matching between the balanced ports, facilitating circuit and system integration. The overall circuit is compact, occupying an area of 0.264 × 0.517 λg2. Measurement results show that, within the 2–3 GHz band, the proposed Marchand balun achieves a return loss lower than 10 dB at both the unbalanced and balanced ports, an insertion loss of approximately 3.5 dB, and isolation between the balanced ports exceeding 15 dB. The fractional bandwidth is 60%. The amplitude and phase imbalances are less than 0.32 dB and 0.8°, respectively. The proposed balun offers a compact size, low cost, and competitive performance on an FR4 substrate, making it suitable for modern wireless communication system applications.