Social working memory (WM) has distinct neural substrates from canonical cognitive WM (e.g., color). However, no study, to the best of our knowledge, has yet explored how social WM develops. The current study explored the development of social WM capacity and its relation to theory of mind (ToM). Experiment 1 had sixty-four 3- to 6-year-olds memorize 1-5 biological motion stimuli, the processing of which is considered a hallmark of social cognition. The social WM capacity steadily increased between 3- and 6-year-olds, with the increase between 4 and 5 years being sharp. Furthermore, social WM capacity positively predicted preschoolers' ToM scores, while nonsocial WM capacity did not; this positive correlation was particularly strong among 4-year-olds (Experiment 2, N = 144).