Background: Despite the potential of captioned videos, limited attention has been paid to the role of vocabulary knowledge (VK) and working memory (WM) in incidental vocabulary learning under different captioning conditions. Objectives: The present study aimed to bridge this gap by assessing VK and WM in incidental vocabulary learning under different captioning conditions. Methods: A total of 101 Chinese primary school students were assigned to one of the three conditions: watch fully captioned videos (n = 33), watch keyword captioned videos (n = 35), and watch videos without captions (n = 33). A vocabulary test measuring recognition and recall of form and meaning served as a pretest, posttest and delayed posttest. The learners also completed tests for VK and WM. Results and Conclusions: The findings supported that both full captions and keyword captions made significant contributions to the incidental learning of form recognition and initial meaning recall and to the retention of form recognition but not of delayed meaning recall. The parameters of breadth and depth of VK and phonological WM impacted incidental vocabulary learning outcomes. The findings emphasized the important role of full captioning and keyword captioning for incidental vocabulary learning and the need to consider the role of VK and WM in incidental vocabulary learning from captioned videos. Takeaways: Captioning is a promising tool for vocabulary learning, but individual differences in VK and WM should be considered to maximize the benefits of captioning on vocabulary learning.