We study the discovery prospects for a doubly charged Higgs boson $H^{\pm \pm }$ in the 2-Higgs doublet model with type-II seesaw at future $e^{+}{e}^{-}$ colliders. Focusing on the three-body channels $e^{+}{e}^{-}\to H^{\pm \pm }{H}_1^{\mp }{H}_1^{\mp }$ and $e^{+}{e}^{-}\to H^{\pm \pm }{H}_1^{\mp }{W}^{\mp }$, we scan the model parameter space subject to theoretical consistency as well as current collider, flavor, and electroweak precision observables. We find that these $2\to 3$ production modes can exceed the conventional pair production rate $e^{+}{e}^{-}\to H^{++}{H}^{--}$, followed by $H^{\pm \pm }\to H_1^{\pm }{H}_1^{\pm }$ and $H_1^{\pm }{W}^{\pm }$ decays, over wide regions, particularly above the $H^{\pm \pm }\to H_1^{\pm }{H}_1^{\pm }$ and $H^{\pm \pm }\to H_1^{\pm }{W}^{\pm }$ thresholds, reaching cross sections up to $\mathcal{O}({10}^2)\mathrm{fb}$ for $\sqrt{s}=500–1500\mathrm{GeV}$. A detector-level analysis of the $4\ell +{\cancel{E}}_T$ signature, including dominant multiboson and top quark backgrounds, shows that discovery sensitivity is achievable for $\sqrt{s}=1000–1500\mathrm{GeV}$ with integrated luminosities in the few ${\mathrm{ab}}^{-1}$ range, even in the presence of realistic systematic uncertainties.

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• Extensions of Higgs sector
• Hypothetical particle physics models
• Phenomenology