AbstractCUPID-Mo is a bolometric experiment to search for neutrinoless double-beta decay ($0ν β β $0νββ) of $^{100}\hbox {Mo}$100Mo. In this article, we detail the CUPID-Mo detector concept, assembly and installation in the Modane underground laboratory, providing results from the first datasets. The CUPID-Mo detector consists of an array of 20 $^{100}\hbox {Mo}$100Mo-enriched 0.2 kg $\hbox {Li}_2\hbox {MoO}_4$Li2MoO4 crystals operated as scintillating bolometers at $∼ 20\hbox { mK}$∼20mK. The $\hbox {Li}_2\hbox {MoO}_4$Li2MoO4 crystals are complemented by 20 thin Ge optical bolometers to reject $α $α events by the simultaneous detection of heat and scintillation light. We observe a good detector uniformity and an excellent energy resolution of 5.3 keV (6.5 keV) FWHM at 2615 keV, in calibration (physics) data. Light collection ensures the rejection of $α $α particles at a level much higher than 99.9% – with equally high acceptance for $γ $γ/$β $β events – in the region of interest for $^{100}\hbox {Mo}$100Mo$0ν β β $0νββ. We present limits on the crystals’ radiopurity: $\le 3~μ \hbox {Bq/kg}$≤3μBq/kg of $^{226}\hbox {Ra}$226Ra and $\le 2~μ \hbox {Bq/kg}$≤2μBq/kg of $^{232}\hbox {Th}$232Th. We discuss the science reach of CUPID-Mo, which can set the most stringent half-life limit on the $^{100}\hbox {Mo}$100Mo$0ν β β $0νββ decay in half-a-year’s livetime. The achieved results show that CUPID-Mo is a successful demonstrator of the technology developed by the LUMINEU project and subsequently selected for the CUPID experiment, a proposed follow-up of CUORE, the currently running first tonne-scale bolometric $0ν β β $0νββ experiment.