Perceptual decisions are biased towards previous stimuli, a phenomenon known as serial dependence. However, the mechanisms of this bias are unknown. For example, it is debated whether serial dependence reflects a perceptual bias or interference of prior stimuli in working memory. We addressed this question by testing whether visual working memory load modulates serial dependence. We presented sequences of Gabors appearing at the same or different locations. At the beginning of each sequence, we presented a display with two lines. Participants reproduced the orientation of each of the three Gabors while holding the orientation of the lines in memory. In a control condition, participants reproduced only the orientation of each Gabor, ignoring the lines. Working memory load caused a repulsive bias in the adjustment responses when the previous Gabor was presented at a different location. In the control condition, Gabors at different locations caused no bias at all. When presented at the same location, however, the bias was attractive for small orientation differences (<45°) and repulsive for larger ones (>45°), resembling typical serial dependence. Under load, this pattern was qualitatively more pronounced. Our results suggest that working memory load may interact with, rather than prevent, serial dependence by making perceptual decisions more vulnerable to biases. [This research was supported by funding from the Swiss National Science Foundation (grant no. 415 PZ00P1_179988 to DP) and the Swiss Government Excellence Scholarship awarded by Federal Commission for Scholarships for Foreign Students FCS (NT). The funders had no role in the study design, data collection and analysis.]