Arresting Caries Treatment (ACT) has been proposed to manage untreated dental caries in children. This prospective randomized clinical trial investigated the caries-arresting effectiveness of a single spot application of: (1) 38% silver diamine fluoride (SDF) with tannic acid as a reducing agent; (2) 38% SDF alone; (3) 12% SDF alone; and (4) no SDF application in primary teeth of 976 Nepalese schoolchildren. The a priori null hypothesis was that the different treatments have no effect in arresting active cavitated caries. Only the single application of 38% SDF with or without tannic acid was effective in arresting caries after 6 months (4.5 and 4.2 mean number of arrested surfaces; p < 0.001), after 1 year (4.1 and 3.4; p < 0.001), and after 2 years (2.2 and 2.1; p < 0.01). Tannic acid conferred no additional benefit. ACT with 38% SDF provides an alternative when restorative treatment for primary teeth is not an option.