The distribution patterns of geographical elements of seed plants from 135 regional floras that cover southern China were used to reassess the extent and boundaries of the tropical zone. The areas for which tropical genera account for >80% of the total genera in the flora are south of 22°30′N in southern and southeastern China, which corresponds closely to the northern boundary of the tropical monsoon forest and rain forest in southeastern China. The line at c. 22°30′N is therefore suggested to be the northern biogeographical boundary of the tropical zone in south and southeastern China. This line exceeds the northern boundary of marginal tropical climate, which implies that the tropical zone could have extended further north in the geological past than it does today. The study supports the suggestion from palaeoecological studies that tropical and subtropical broadleaved evergreen forests in eastern China shifted north during the mid-Holocene. It also shows that there are climatic and biogeographical disparities between southeastern and southwestern China due to their different topography and geology. •22°30′N for the northern biogeographical boundary of the tropical zone in China is proposed.•The northward shift of tropical forest in eastern China in the geological past is supported.•Biogeographical disparities between southeastern and southwestern China is revealed.