The nucleus reuniens (Re) of the ventral midline thalamus is known to be a critical anatomical link between the hippocampus (HPC) and the medial prefrontal cortex (mPFC). Consistent with this anatomical connectivity, the Re has been shown to be crucial for HPC-mPFC oscillatory synchrony. Moreover, Re inhibition consistently results in spatial working memory (SWM) deficits. Together, these results suggest that SWM requires HPC-mPFC synchrony via the Re. In spite of these findings, an understanding of how the Re contributes to the encoding, maintenance, and retrieval of spatial information during a SWM task is lacking. To address this issue, we trained rats to perform a SWM-dependent delayed-non-match-to-position (DNMP) task in a T-maze. Using optogenetic inhibition of Re activity, we demonstrated that Re suppression during the sample phase, but not the delay or choice phase, significantly decreased choice accuracy. We conclude that the Re contributes to the encoding of spatial information during working memory.