Altanbulag (Mongolian: Алтанбулаг, golden spring) is a sum (district) of Selenge Province in northern Mongolia. It is in located about 25 km from the provincial capital of Sükhbaatar, on the border with Russia. Altanbulag is the location of the Altanbulag Free Trade Zone (Алтанбулаг худалдааны чөлөөт бүс, Altanbulag khudaldaany Chölööt büs).
Today, the town is known as Altanbulag in both the Mongolian and Buryat languages.
During the Qing dynasty, the town was known as Maimachen (Маймачен) in Russian, which derived from the Chinese name Mǎimàichéng (買賣城, trading town). In Mongolian, it was known as Övör Khiagt (Өвөр Хиагт, South Kyakhta).