We present a detailed photometric and spectroscopic analysis of DD CMa, based on published survey photometry and new spectroscopic data. We find an improved orbital period of P o= 2.0084530 (6) days. Our spectra reveal Hβべーた and Hαあるふぁ absorptions with weak emission shoulders, and we also find a color excess in the Wide-field Infrared Survey Explorer multiband photometry, interpreted as signatures of circumstellar matter. We model the V-band orbital light curve derived from the ASAS and ASAS-SN surveys, assuming a semidetached configuration and using the mass ratio and temperature of the hotter star derived from our spectroscopic analysis. Our model indicates that the system consists of a B2. 5 dwarf and a B9 giant of radii 3.2 and 3.7 R⊙, respectively, orbiting in a circular orbit of radius 6.75 R⊙. We also found M c= 1.7±0.1 M⊙, T c= 11,350±100 K, and M h= 6.4±0.1 M⊙, T h= 20,000±500 K, for the cooler and hotter star, respectively. We find broad single emission peaks in Hαあるふぁ and Hβべーた after subtracting the synthetic stellar spectra. Our results are consistent with mass exchange between the stars and suggest the existence of a stream of gas being accreted onto the early B-type star.