Despite learning English formally in primary classrooms, most of the Malaysian primary learners are incapable of conveying their ideas accurately in the target language since they have limited vocabulary. Action research based on the Kemmis and McTaggart Action Research Model was carried out to alleviate the vocabulary learning challenges. Thus, the study aims are to investigate the effects of the Poly Category Mind Map for vocabulary development of third-year students. Thirty students from the third-year suburban school in Mersing, Johor participated in the study. Vocabulary pre-test and post-tests used as the instruments of collecting the data. The results of the post-test at the end of the second cycle showed a significant difference between the mean scores before and after using Poly Category Mind Map to develop vocabulary (p>0.05). The use of Poly Category Mind Mapping helped students in understanding and remembering the words, their meaning, and spelling precisely. The findings concluded that using the features of Poly Category Mind Map, which are the pictures, keywords, and grouping of words aided the students in learning vocabulary. This study hopes to encourage students, teachers, and curriculum designers to integrate Poly Category Mind Mapping strategy in English classrooms. Researchers can also utilise the Poly Category Mind Mapping in learning skills of the English Language such as reading and writing for further studies.