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In this paper, the design and hardware implementation of a squared open-loop resonator (SOLR)-based microstrip diplexer with high isolation, low insertion loss, and high selectivity are introduced. We employed four SOLRs, with each pair of coupled SOLRs used to build a high selectivity bandpass filter (BPF). To assemble the proposed diplexer, the designed BPFs are linked together via a T-junction combiner that is matched to the two filters and the antenna port. For transmit and receive modes, the proposed diplexer has two resonance frequencies of ft = 1.81 GHz and fr = 2.03 GHz, respectively achieving a small frequency space ratio of R = 0.114. The simulated structure exhibits good insertion losses of about 1.98 dB and 1.9 dB for the two channels, respectively, with fractional bandwidths of 2.25% at 1.81 GHz and 3% at 2.03 GHz. For 1.81 GHz and 2.03 GHz, the simulated isolation values are 58 dB and 46 dB, respectively. While the fabricated structure exhibits better insertion losses of about 1.25 dB and 1.22 dB at the measured transmit and receive frequencies of 1.801 GHz and 2.001 GHz, respectively, with smaller fractional bandwidths of 2.23% at 1.801 GHz and 2.98% at 2.001 GHz. For 1.801 GHz and 2.001 GHz, the measured isolation values are 48.99 dB and 57.02 dB, respectively. [ABSTRACT FROM AUTHOR]