Collapsible soils experience significant volume decrease due to the increase of soil moisture content, with or without an increase in the in-situ stress level. These soils form large parts of North and South America, Eastern Europe, China, Central Asia, Northern and Southern Africa, Russia, Egyptian western dessert, and the continuous deposit from North China to South-East England. As human activities continue to increase in these regions, geotechnical engineers must learn how to deal with these problematic soils. Foundations on collapsible soils suffer from sudden settlement, which may contribute to serious damage or catastrophic failure due to inundation. For relatively light structures, the use of shallow foundations combined with soil replacement or treatment may constitute economical designs. For heavy structures, pile is perhaps the best of alternative available types of foundation. This subject of significant practical interest has received little attention from the researchers mainly due to the complexity in conducting experimental study. Furthermore, numerical modeling is difficult at best due to the complexity associated in describing the behavior of collapsible soil during inundation. Analytical modeling is not suitable in this respect, as soil grains in collapsible soil undergo radical rearrangements during inundation. In the literature, no design theory can be found to predict the negative skin friction on pile foundation due to inundation of collapsible soil. Present study presents a numerical model, which is capable to incorporate the effect of inundation of collapsible soil on an axially loaded vertical pile. It employs the theories of unsaturated soil mechanics; including the Soil-Water Characteristic Curve (SWCC) to include the effect of soil suction reduction resulted from the progressive inundation, from two different aspects: change in soil properties, and irrecoverable soil volume change. The proposed numerical model is used to predict negative skin friction exerted on the pile during inundation of collapsible soil surrounding the pile. The numerical model is validated by comparing the numerical results and the experimental data from the literature. Moreover, another numerical modeling procedure is also proposed to design the pile (i.e., length and diameter) in collapsible soil, provided that the indirect load due to negative skin friction is known. An extensive numerical investigation is carried out to identify the parameters (e.g., collapse potential, radius of wetting, pile roughness, etc.) influencing the negative skin friction acting on a pile during inundation. Based on the numerical results, analytical models that can be directly used to predict the indirect load due to negative skin friction are established for both directions (i.e., from bottom and top) of inundation. Design procedures that can provide adequate positive skin friction and pile capacity in accommodating indirect load due to negative skin friction, are proposed to design the length and diameter of a single pile in collapsible soil subjected to inundation. Present study is useful in reductions in the costs of construction, litigation and remediation in geotechnical engineering practice.