The design and synthesis of quasi-elliptic triple-mode filter with three transmission poles and three transmission zeros are presented in this paper. The transfer and reflection filtering functions are obtained to get the even- and odd-mode admittance. The synthesized admittances give the even- and odd-mode networks, routing structure and coupling matrix of the filter. The microstrip prototype of the quasi-elliptic triple-mode filter is designed and realized to prove the feasibility of the approach. The filter is realized by having a capacitive coupling between the input and the output of a proposed triple mode resonator. The results show an excellent agreement with the theories.