Working memory (WM) has recently been described as internally directed attention, implying that internally activated WM content should impact behavior exactly like a visually perceived and attended stimulus. We tested whether holding a color-word in WM, rather than attending to it in the external environment, can produce interference in a color-discrimination task, mimicking the classic Stroop effect. Over three experiments, the “WM Stroop effect” recapitulated core properties of the classic “attentional” Stroop effect, displaying equivalent congruency effects, additive contributions from stimulus- and response-level congruency, and susceptibility to modulation by the proportion of congruent and incongruent trials. Moreover, WM maintenance was inversely related to attentional demands during the WM delay, with poorer memory following incongruent than congruent trials. Together, these results suggest that WM and attention rely on the same resources and operate over the same representations.