Individuals with hoarding disorder tendencies can be identified as teenagers and get worse as adults.This study aimed to test the Declutter Challenge intervention in reducing hoarding disorder tendencies using a quasi-experimental approach.Thirty-one teenagers involved in this study screened by the hoarding tendencies screening tool administered online who have hoarding disorder tendencies at moderate to high level.The selection of participants into control and experiment groups was conducted nonrandomized.Data collection methods used observation, interviews, and the Saving Inventory-Revised scale (α = 0.93 and test-retest reliability r = 0.86), Hoarding Rating Scale (α = 0.92), and Clutter Image Rating (α = 0.82).The t-test was used to examine the differences in saving inventory-revised scale scores during the pretest, posttest and follow-up both for experimental and control groups.The results of the experiment group after being treated reported significantly lower levels of hoarding disorder tendencies than the control group ( p< 0.05).There was a significant difference in gained scores between the control group and the experimental group.Therefore, it can be concluded that the Declutter Challenge could help reduce hoarding disorder tendencies.Moreover, a qualitative analysis indicated that several behavioral patterns were found related to hoarding indicators.Besides, the participants felt relieved and happy because they were able to sort and put away items wisely.