In this communication, a differentially fed wideband slot antenna on a single layer with endfire radiation and filtering response under multi-resonant modes is proposed. By using the λ/4 slot radiator with its opened terminal at one side, the endfire-radiated fields of the slot antenna is initially formed with compact dimension and endfire radiation. After that, the λ/4 slot radiator with two distinctive portions of different slot widths is presented and investigated. The results demonstrate that the resonant frequencies of the first two odd-order modes could be progressively reallocated in proximity to each other as the length and width ratios of these two portions are properly selected. Thus, the bandwidth in input impedance of the entire differentially fed slot antenna could be significantly improved under operation of dual-resonant modes. Next, an extra microstrip-line resonator is introduced in the feeding part so as to produce a third resonant mode and improve its frequency selectivity in the wide operating band. To validate these predicted performances, a filtering slot antenna operating from 3.33 to 4.50 GHz is in final fabricated and tested. The measured results exhibit good impedance matching property, flat radiation gain of about 5 dBi, and improved endfire radiation characteristic with the front-to-back ratio (FBR) better than 9.0 dB in the operating band. Both the simulated and measured results are provided with good agreement.