Dechen Dzong stands on the site where Lam Zhang Dorji Dragpa built a monastery in the 12th century. It is said that the Zhemgang dzongkhag derives its name from the monastery, which was known as ‘Zhangang’ (meaning ‘immeasurable mound’) after its founder. Lam Zhang was a scholar-sage of Drukpa Kagyud School who visited Bhutan from Zhamling in Tibet to propagate Buddhism. At that time, Zhemgang region was known as Khengrig Namsum. In the 17th century, the dzong was built at the site of the monastery and it played an important role in the history of then Khengrig Namsum. Dechen Yangtse dzong has witnessed some of the major historical events such as triumphs, intrigues, civil strife and struggle for power amongst the local chieftains and final consolidation under the central government.
Zhemgang dzong has important political and religious significance in the history. Until the 17th century, Kheng region was divided into multiple kingdoms and the local chieftains were trying to gain supremacy over each other. Nyakhar Dung was notably the most powerful chieftain in the region. In the 17th century, Zhabdrung Ngawang Namgyal had already consolidated his powers in the eastern region and was now concentrating on the unification in the central region. He appointed Trongsa Penlop Minjur Tenpa to do the job. Tunglabi Dung, an arch rival of Nyakhar Dung, sought from the Trongsa Penlop, who intervened and defeated the Nyakhar Dung’s force. Other local chieftains surrendered and thus, the whole region of Kheng came under Penlop. In 1655, a single storied dzong was built there. Khengrig Namsum was unified with the Sharchog Khorlo Tsibgyed or the whole of the eight eastern districts, and came under the direct control of the Zhabdrung’s central government.
It is said that a powerful Khaling overlord called Lango (a mythical creature who was half human and half bull) plotted against and killed Lam Zhang, the founder of the monastery. Lam Zhang was killed in the Trong village, which is believed to be cursed to this day because of assassination. It is believed that people of Trong will always be poor. The oldest religious structure in Zhemgang is a gold plated six-inch image of Lam Zhang in the Goenkhang that somehow managed to escape two fire incidents that destroyed the dzong and the authentic chronicles and manuscripts that had been preserved herer. The dzong was renovated in 1963 when Zhemgang was created as a separate dzongkhag, under the command of His late Majesty, King Jigme Dorji Wangchuk and was renamed as Dechen Yangtse or Druk Dechen Yangtse dzong. In mid-1980s, the dzongkhag administration headquarters was separated from the old dzong and shifted to a separate two-storied building.