The present study investigates the rainfall-induced long-runout landslide that occurred in the Achanakkal area of the Nilgiris district in the Tamil Nadu state of India on November 11, 2009. The landslide damaged a house located on the runout path and killed its eight residents. The slide also damaged a road network, affecting traffic for several weeks. The main objective of the study is to understand the failure mechanism associated with Achanakkal 2009 landslide. This study is composed of a field investigation, laboratory experiments, and a numerical simulation of the landslide. Based on the field investigation including a topographical survey, a borehole investigation, a geophysical test, and laboratory classification tests, the slope geometry was identified and the subsoil was characterized. Ring shear tests were performed to estimate the shear strength parameters such as peak and residual friction angle (φp and φm) and steady-state shear resistance (τss). Numerical simulation of the landslide was carried out using the LS-RAPID landslide simulation model by incorporating the shear strength parameters obtained from the ring shear tests. The simulation results reproduced the failure mechanism of the Achanakkal landslide and matched the runout distance, landslide boundary, and distribution of debris mass that were observed from the topographical survey well. The analysis results revealed that the rainfall infiltration due to 5 days' antecedent rainfall of 594 mm; a very large amount of daily rainfall, 820 mm, on the event day; and accumulation of rainwater on the source area charging from the above plateau due to blocking of drainage culverts increased the soil saturation. An increase in saturation level of the soil developed the pore water pressure in the source area. The development of pore pressure to ru=0.5 significantly reduced the shear resistance of the soil at the slip surface from the peak value to the residual value along with shear displacement was the possible cause for triggering of Achanakkal landslide. The analysis also revealed that the simulation model initiates the landslide at a smaller pore water pressure ratio than the limit equilibrium method of stability analysis due to progressive failure. © 2019 American Society of Civil Engineers.