Sonchus arvensis (Asteraceae) is a traditional medicinal herb. The aerial parts are a rich source of vitamins, essential amino acids, and minerals, which may help in treatment of fever, inflammation, detoxication, and blood circulation (Li et al. 2018). In October 2018, typical rust symptoms were observed on S. arvensis leaves and stems in Buner district, Malakand division, Khyber PakhtunKhwa Province, Pakistan (34.39°N; 72.61°E). Almost 40% of leaves and stems of five S. arvensis plants displayed severe rust. The specimens were examined microscopically and compared with type specimen description in published literature (Dietel 1908; Hiratsuka et al. 1992). The fungus was identified as Puccinia crepidis-japonicae based on the characteristic of spore morphology and phylogenetic study based on the internal transcribed spacer (ITS) and large subunit (LSU) sequence data. Uredinia were amphigenous, rounded, or somewhat elliptical, naked, small patches, equally spread, brown. Urediniospores (n=30) were globose to ellipsoid, brownish yellow and measured 22.4-24.7 × 20.2-22.1 µm. Urediniospore walls were brownish orange and finely echinulate and 1.7 to 2.1 µm thick with 2 to 3 germ pores. Telia were amphigenous, rounded or elliptic, scattered, dark brown to blackish. Teliospores (n=30) were ellipsoid, subglobose or long ellipsoid, rounded at both ends, not thickened at apex, warted, reddish brown, and measured 31.3-39 × 24.6-26.8 µm. Teliospore walls were reddish-black and about 1.5-2.5 µm thick, and the pedicles were short, hyaline, fragile, become tapered toward apex, and measured 14.4-18.7 × 4.7-9 µm. DNA was extracted from urediniospores, and the combined region of ITS and LSU (28S) were amplified using Rust2Inv (forward primer) and LR6 (Reverse primer) according to the protocol outlined by Aime (2006). A BLASTn search (http://www.ncbi.nlm.nih.gov) showed that the combined ITS and LSU region shared 99% identity (792/804 bp) to the P. crepidis-japonicae accessions (KY798395 from Hawaii, USA) with 100% query cover. The resulting sequence was deposited in GenBank (Accession No. MN093335). Both morphological and molecular characteristics indicatethat this species was P. crepidis-japonicae. To test pathogenicity and fulfill the Koch's postulates, a urediniospore suspension (1 × 104 spores/ml) was sprayed on three 6-week-old plants of S. arvensis, and one as negative control, incubated at 22-24°C. Uredinia were observed on the leaves after 10 days of inoculation, whereas the control plants remained symptomless. Microscopic examination confirmed that the symptoms on plants obtained from the field and greenhouse inoculations were morphologically identical. This fungus has been observed previously on Crepis japonica in China, Hong Kong, Japan, Korea, and Taiwan, on Prenanthes spp. in China and on Youngia tenuifolia and Y. fusca in China (Farr and Rossman 2021). Pereira et al. (2002) suggested that P. crepidis-japonicae may play a significant role as a biocontrol agent against its weed host. To the best of our knowledge, there are no other reports of this fungus on any other hosts in Pakistan. The specimen has been vouchered in LAH Herbarium, Department of Botany, University of the Punjab, Lahore, Pakistan (LAH36343). This is the first report of P. crepidis-japonicae on S. arvensis as a new host from Pakistan.