Taiwan, with 101,000 arrivals, slipped from third to sixth place, according to data from the General Statistics Office.
Tourism insiders attributed the surge to Cambodia's close proximity, affordable travel costs and convenient transport connections.
Air Cambodia currently operates 10 flights a day to Vietnam and has expanded its network to include services from Phnom Penh to Cam Ranh, Da Nang and Phu Quoc.
The new Phnom Penh-Cam Ranh service is twice weekly and a Siem Reap-Cam Ranh is scheduled to launch in mid-2026.
As ASEAN members the two nations have mutual visa waivers.
Vietnam received 2.5 million foreign visitors in January, its highest monthly number ever, thanks to relaxed visa policies and extensive tourism campaigns.
South Korea overtook mainland China to become the top market with nearly 490,000 visitors. There were 460,000 Chinese visitors.
Russia ranked fourth, followed by the U.S.