Ust-Barguzin (; , Bargajanai Adag) is an urban locality (an urban-type settlement) in Barguzinsky District of the Republic of Buryatia, Russia, located on the shore of Lake Baikal at the mouth of the Barguzin River and northeast of Ulan-Ude, the capital of the republic. As of the 2010 Census, its population was 7,173.
It was founded in 1666 by a Cossack detachment under Gavril Lovzov.
Within the framework of administrative divisions, the urban-type settlement (inhabited locality) of Ust-Barguzin, together with eight rural localities, is incorporated within Barguzinsky District as Ust-Barguzin Urban-Type Settlement (an administrative division of the district). As a municipal division, Ust-Barguzin Urban-Type Settlement is incorporated within Barguzinsky Municipal District as Ust-Barguzin Urban Settlement.
Ust-Barguzin has a subarctic climate (Köppen climate classification Dwc) with severely cold winters and mild summers. Precipitation is quite low and is significantly higher in summer than at other times of the year.
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