Phomopsis longicolla is an important seed-borne pathogen affecting soybean. In this research, transcription elongation factor 1-α (TEF1-α) was identified as a suitable target for detecting P. longicolla using Loop-mediated isothermal amplification (LAMP) method. The specificity of the method was tested against 54 P. longicolla isolates, 13 oomycetes isolates and 28 other fungal isolates. With the addition of hydroxynaphthol blue (HNB) prior to amplification, a sky-blue color was only observed in the presence of P. longicolla, whereas other isolates showed no color change. The method was sensitive enough to detect as little as 100 pg/uL fungal DNA. In addition, the assay also detected P. longicolla from diseased soybean tissues and residues from different origins. The results demonstrated that the LAMP assay provides a rapid and sensitive tool for detecting P. longicolla from plant tissues.