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An innovative single-layer, differentially fed triband filtenna is presented. It is formed with a main patch, which consists of two interdigitated-coupled driven patches that are approximately mirror-symmetric and four parasitic patches. The strong coupling between the two driven patches facilitates the excitation of the TM $_{\mathbf {1,0}}$ , TM $_{\mathbf {1,2}}$ , TM $_{\mathbf {1,4}}$ , and antiphase TM $_{\mathbf {2,0}}$ modes of the entire main patch and the consequent very small electrical size of the filtenna. Two pairs of open-ended branches are loaded on the main patch to improve the selectivity of the middle band. A pair of split-ring slots are etched on the main patch and four parasitic shorted patches are loaded near it to introduce additional in-band resonances and out-of-band radiation nulls simultaneously, which improve the bandwidth and selectivity of the lower and higher bands, respectively. A prototype was fabricated, assembled, and tested. Its electrical size is $0.31\lambda _{\mathbf {L}} \times 0.28\lambda _{\mathbf {L}} \times 0.031\,\,\lambda _{\mathbf {L}}$ , $\lambda _{\mathbf {L}}$ being the free-space wavelength corresponding to the lower band’s center frequency. The experimental results, in agreement with their simulated values, demonstrate operation in three bands: 2.4–2.487, 3.72–3.93, and 5.57–5.88 GHz, with uniform polarization and similar broadside radiation performance. They also show excellent low cross-polarization levels, high common-mode noise immunity, and good filter properties over all three bands.