To better understand the influence of monsoonal climate and transport of atmospheric mercury (Hg) in southwestern China, measurements of total gaseous mercury (TGM, defined as the sum of gaseous elemental mercury, GEM, and gaseous oxidized mercury, GOM), particulate bound mercury (PBM) and GOM were carried out at Ailaoshan Station (ALS, 2450 m a.s.l.) in southwestern China from May 2011 to May 2012. The mean concentrations (± standard deviation) for TGM, GOM and PBM were 2.09 ± 0.63 ng m−3, 2.2 ± 2.3 pg m−3 and 31.3 ± 28.4 pg m−3, respectively. TGM showed a monsoonal distribution pattern with relatively higher concentrations (p = 0.021) during the Indian summer monsoon (ISM, from May to September) and the East Asia summer monsoon (EASM, from May to September) periods than that in the non-ISM period. Similarly, GOM and PBM concentrations were higher in the ISM period than in the non-ISM period. This study suggests that the ISM and the EASM have a strong impact on long-range and transboundary transport of Hg between southwestern China and South and Southeast Asia. Several high TGM events were accompanied by the occurrence of northern wind during the ISM period, indicating anthropogenic Hg emissions from inland China could rapidly increase TGM levels at ALS due to strengthening of the EASM. Most of the TGM and PBM events occurred at ALS during the non-ISM period. Meanwhile, high CO concentrations were also observed at ALS, indicating that a strong south tributary of westerlies could have transported Hg from South and Southeast Asia to southwestern China during the non-ISM period. Consequently, southwestern China is an important anthropogenic source region of ALS during the ISM period. The biomass burning in Southeast Asia and anthropogenic Hg emissions from South Asia should be the source of atmospheric Hg in remote areas of southwestern China during the non-ISM period.