A previously unseen maser in the J = 1-0 transition of HCO+ has been detected by the Combined Array for Millimeter-wave Astronomy (CARMA). A subarcsecond map was produced of the 2 arcmin^2 region around DR21(OH), which has had previous detections of OH and methanol masers. This new object has remained undetected until now due to its extremely compact size. The object has a brightness temperature of >2500 K and an FWHM linewidth of 0.497 km s^-1, both of which suggest non-thermal line emission consistent with an unsaturated maser. This object coincides in position and velocity with the methanol maser named DR21(OH)-1 by Plambeck & Menten. No compact HCO+ emission was present in the CARMA data toward the other methanol masers described in that Letter. These new results support the theory introduced in Plambeck & Menten that these masers likely arise from strong outflows interacting with low mass, high density pockets of molecular gas. This is further supported by recent observations of a CO outflow by Zapata et al. that traces the outflow edges and confirms that the maser position lies along the edge of the outflow where interaction with molecular tracers can occur.