A soil suitability map was developed for teak (Tectona grandis L.f.) plantations in northern Lao People’s Democratic Republic based on 110 sample sites located along a 40 km transect line running northwest–southeast in southwestern Luang Prabang Province, 63 of which included teak stands. The soils were observed to a depth of 1 m using a soil auger at 59 sites and by obtaining a soil profile at 51 sites, and the soil physicochemical properties were summarized for the 0–20 cm depth layer. In addition, the height and diameter at breast height of the teak trees and stand age were measured in a 20 × 20 m plot at each teak stand. The stand age ranged from 3 to 31 years, and the average dominant tree height ranged from 5.1 to 27.7 m. The site index (SI), which was defined as the dominant tree height at base age (20 years), was computed based on a height–growth model using the Richards function and ranged from 12.6 to 27.0. A predictive model SI based on soil physicochemical properties, site conditions, and terrain characteristics was developed using the decision tree method, which showed that CN ratio, electrical conductivity, convergence index, coarse sand fraction, slope gradient, and 22 additional factors contributed to the SI. This model was then used to construct a soil suitability map, which showed that suitable land covered approximately 50% of the study area, 8% of which was optimal land, while unsuitable land covered approximately 30% of the area.