Under-reamed piles are generally bored cast in-situ concrete piles having single or multi-bulb by enlarging the stem of the pile. Due to its enlarged bearing surface of the bulb, under-reamed piles have a greater load carrying capacity compared to pile with uniform cross-sectional area. This study presents the numerical investigations on single and group under-reamed piles in soft cohesive soil to understand the deformation behavior of under-reamed pile foundation under different loading conditions. Three different configurations of under-reamed bulbs were considered in this study. A three-dimensional finite element analysis using PLAXIS 3D has been conducted assuming Mohr–coulomb model to simulate the behavior of soil and the performance of the single and group under-reamed piles with one bulb subjected to both compression and tension were studied. From the analysis results, it was observed that use of under-reamed pile is more suitable in tension than in compression in cohesive soil and an enhancement of 147% in tensile capacity was found for single under-reamed pile. Also bottom position of bulb was found to be more effective specially under tensile load. Group efficiency value depends on many factors like number of piles in the group, pile spacing, configuration of under-reamed bulb and loading condition. Under compression, optimum pile spacing in the pile group was observed as 2.5 times the diameter of the under-reamed bulb while maximum efficiency under tensile load was found at the pile spacing of 3.5 times the diameter of the under-reamed bulb.