We report the detection of interstellar methoxymethanol (CH_3_OCH_2_OH) in Atacama Large Millimeter/submillimeter Array (ALMA) Bands 6 and 7 toward the MM1 core in the high-mass star-forming region NGC 6334I at ~0.1-1" spatial resolution. A column density of 4(2)x10^18^cm^-2^ at T_ex_=200K is derived toward MM1, ~34 times less abundant than methanol (CH_3_OH), and significantly higher than predicted by astrochemical models. Probable formation and destruction pathways are discussed, primarily through the reaction of the CH_3_OH photodissociation products, the methoxy (CH_3_O) and hydroxymethyl (CH_2_OH) radicals. Finally, we comment on the implications of these mechanisms on gas-phase versus grain-surface routes operative in the region, and the possibility of electron-induced dissociation of CH3OH rather than photodissociation.