AbstractMotivationPrevious studies on the effectiveness of capacity building training, drawn from the experience of traditional donors, have been limited in their understanding the impact of the diversity of trainees and the diversity of training on the effectiveness of training. This is particularly problematic given the increasing prevalence of such training and the emergence of new donors with different approaches.PurposeThis article investigates the effectiveness of capacity‐building training across different participants and different training types, through the case of an emerging donor, the Republic of Korea (Korea). Korea is distinctive and under‐researched as a country relatively late to join the OECD's Development Assistance Committee (DAC), with several policies explicitly locating capacity‐building training as a diplomatic strategy. Korea has made repeated efforts to diversify participants and types of training to gain a comparative advantage in international development and co‐operation.Methods and ApproachThe study constructs a generalized linear mixed model (GLMM) to analyse the effectiveness of training across layers of individual diversity (gender, rank, level of national income and extent of democracy) and group diversity (types of trainings). It draws on survey responses from 789 government officials in low‐ and middle‐income countries (LMICs).FindingsTraining effectiveness differs for diverse participants and training types. Effectiveness tends to decrease as job rank and national income rise, while it increases with the level of democracy. Training that fosters diverse interactions and experiences, such as multi‐country training and invitational training, tends to be more effective than other types.Policy ImplicationsOur results suggest implementing more collaborative and customized training to suit the heterogeneous needs of diverse participants. The results emphasize the need to develop and operate various training types that promote diverse interactions and experiences among participants.