Chinese hackberry (Celtis sinensis Pers.) is an adaptable species widely growing in southern China. The symptoms of canker on stems of seedlings were discovered mid-July 2017 in Shuyang, Jiangsu Province. The diseased portions of the stems were dark brown due to discoloured xylem. Some seedlings showed symptoms of wilting, leaf fall, twig dieback, and tissue discolouration. The outbreak period was concentrated in July and August, suggesting that the disease spread during summer months. Possible fungal causal agents were isolated from naturally infected canker tissue and discoloured xylem. The isolate from xylem tissue with a high frequency (> 50%) was named Ls7 type. Pathogenicity tests were carried out on 4-year-old seedlings. The symptoms of canker began to develop 20 days after inoculation with Ls7 isolate and by day 35, there were dark, enlarged longitudinal lesions. A phylogenetic tree of the isolate was developed using the internal transcribed spacer, elongation factor-1α (tef1-α), β-tubulin gene (TUB) and RNA polymerase II subunit primer genes (RPB2). Based on morphological features and phylogenetic information, the pathogen was identified as Lasiodiplodia pseudotheobromae. This is the first report of L. pseudotheobromae causing canker on Chinese hackberry stems in China.