In this article, a new compact triple-mode filter with wide stopband is proposed on half-mode substrate integrated waveguide (HMSIW) with loaded slots. The loaded slots not only make the TE₂₀₁ mode shift toward the TE₁₀₁ mode, but also introduce an extra resonant mode to achieve a compact triple-mode filter. In addition, the wide stopband is realized because the TE₂₀₂ mode keeps nearly unchanged as the TE₂₀₁ mode decreases to a much lower frequency by virtue of loaded slots. The working principle and design method are given in this context. All these triple resonant modes can be controlled as required. To verify the design concept, a prototype with only 0.23λ_g² area is designed and the upper stopband can be extended to 1.87f₀ with rejection level of 25 dB.