Memory-guided behavior requires maintenance of task-relevant information without sensory input, but the underlying circuit mechanism remains unclear. Calcium imaging in mice performing a delayed Go or No-Go task revealed robust delay activity in dorsomedial prefrontal cortex, with different pyramidal neurons signaling Go and No-Go action plans. Inhibiting pyramidal neurons by optogenetically activating somatostatin- or parvalbumin-positive interneurons, even transiently during the delay, impaired task performance, primarily by increasing inappropriate Go responses. In contrast, activating vasoactive intestinal peptide (VIP)-positive interneurons enhanced behavioral performance and neuronal action plan representation. Furthermore, while endogenous activity of somatostatin and parvalbumin neurons was strongly biased toward Go trials, VIP neurons were similarly active in Go and No-Go trials. Somatostatin or VIP neuron activation also impaired or enhanced performance, respectively, in a delayed two-alternative forced-choice task. Thus, dorsomedial prefrontal cortex is a crucial component of the short-term memory network, and activation of its VIP neurons improves memory retention.