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Plantago lanceolata has been increasingly cultivated in recent years as a forage crop due to its high feeding quality, good adaptability and medicinal benefits. Between 2015 and 2023, Alternaria leaf spot (ALS), appearing as reddish‐brown necrotic spots with a greyish white to pale brown centre, was commonly observed in two forage germplasm nurseries of P. lanceolata of Sichuan Province, China. Initially, 139 Alternaria strains were isolated from leaf lesions, and then identified using the identity of rDNA internal transcribed spacer (ITS) sequences in the NCBI database and the similarity of morphological characteristics. By pure culture technique, six representative isolates were finally obtained for the present study. Based on both multilocus phylogeny (SSU, LSU, ITS, gapdh, rpb2, tef1, endoPG, OPA10‐2, alt a 1, KOG1058 and KOG1077) and morphology, four Alternaria species were identified: three new records of A. alternata, A. arborescens species complex and A. gossypina on this host in China, belonging to section Alternaria, and a new species of section Porri, namely, A. lanceolatae sp. nov. Pathogenicity tests by inoculation on both detached leaves and whole plants revealed that A. lanceolatae was the most important causal agent of ALS, while the other three species were nonpathogenic to P. lanceolata. This study characterized a novel pathogenic Alternaria species of section Porri from P. lanceolata in China and could contribute as useful research for possible ALS management. [ABSTRACT FROM AUTHOR]