Vietnam is deemed particularly vulnerable to the adverse impacts of environmental changes caused by climate change effects. The country’s climate change policy acknowledges education as an essential element of the national response to environmental insecurity threats. To be effective, environmental education (EE) may be mainstreamed across multiple subjects and cross-subject coherence is essential. But how applicable are these claims to the Vietnamese high school education? A growing interest in EE and recognition of the challenges inherent in addressing this issue creates an opportunity to conduct a systematic and comprehensive review to scrutinize how it is mainstreamed at the high school level. Using the cases of Grade-10, Grade-11, and Grade-12 curricula, this paper investigated the extent of coherence in EE across high school subjects. A two-pronged qualitative approach is applied using Qualitative Document Analysis of relevant education policies and subject syllabi, combined with expert interviews. Findings show that school subjects experience different degrees of EE coherence, currently being highest in Geography, followed by Biology and poorly represented in other science subjects. Even though EE is formally embedded in several discipline textbooks, there remains a series of barriers to implementing EE in high schools.