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 $P_mathrm{o}= 2.0084530 pm 0.0000006 ~mathrm{d}$. Our spectra reveal H$beta$ and H$alpha$ absorptions with weak emission shoulders and we also find color excess in the WISE 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 B 2.5 dwarf and a B 9 giant of radii 3.2 and 3.7 $mathrm{R_{odot}}$, respectively, orbiting in a circular orbit of radius 6.75 $mathrm{R_{odot}}$. We also found $M_{mathrm{c}} = 1.7 pm 0.1 ~mathrm{M_{odot}}$, $T_{mathrm{c}} = 11350 pm 100 ~mathrm{K}$ and $M_{mathrm{h}} = 6.4 pm 0.1 ~mathrm{M_{odot}}$, $T_{mathrm{h}} = 20000 pm 500 ~mathrm{K}$, for the cooler and hotter star, respectively. We find broad single emission peaks in H$alpha$ and H$beta$ 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.
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