  The Higgs sector of the standard model can be extended by introducing an
$SU(2)_L$ Higgs triplet $\Delta$ to generate the tiny neutrino masses in the
framework of type-II seesaw mechanism. In this paper, we study the pair
production of the introduced Higgs triplet at future $e^{-}p$ colliders. The
corresponding production cross sections via vector boson fusion process at
FCC-ep and ILC$\otimes$FCC are predicted, where the production of a pair of
doubly charged Higgs is found to be dominant and then used to investigate the
collider phenomenology of the Higgs triplet. Depending on the size of the Higgs
triplet vacuum expectation value, the doubly charged Higgs may decay into a
pair of same-sign charged leptons or a pair of same-sign $W$ bosons. In order
to explore the discovery potential of the doubly charged Higgs at future
$e^{-}p$ colliders, we discuss these two decay scenarios in detail and show
respectively the detection sensitivity on the mass of the doubly charged Higgs.
