As the population ages, so does the prevalence of dementia, a condition in which timely advance care planning (ACP) is recommended. This study explored the barriers and enablers of ACP for Australian GPs caring for people with dementia. Semi-structured qualitative telephone interviews informed by the theoretical domains framework (TDF) were conducted with 16 GPs from the south-east region of metropolitan Melbourne. The most prevalent domains of the TDF were: (1) environmental context and resources; (2) beliefs about consequences; and (3) social/professional role and identity. Further thematic analysis found that: (1) ACP was felt to be within the scope of general practice, but more so before the onset of dementia because lack of confidence in capacity assessment acted as a barrier once a diagnosis was made; (2) beliefs about the perceived benefits of ACP motivated GPs to engage patients in ACP; and (3) doctors felt that patients were often reluctant to discuss ACP, especially in the time-pressured context of the standard consultation. This study's use of the TDF provides direction for potential interventions to alleviate challenges faced by Australian GPs with ACP in dementia. Improving ACP in general practice may require a focus on public health campaigns to educate patients about the benefits of ACP before the onset of dementia. GPs' difficulty assessing a patient's decisional capacity may be addressed through skills development workshops.