Complementary medicine (CM) use is prevalent among cancer patients, yet it is often not assessed by oncology healthcare providers (HCPs). The purpose of this study was to evaluate oncology HCPs' knowledge, attitudes, and practices surrounding CM use before and after the implementation of a practice guideline focusing on standardizing assessment and documentation of CM. Oncology HCPs across a provincial cancer agency were invited to participate in the study. The implementation strategy included an initial education session for HCPs and standardized CM assessment forms. Pre-post surveys assessing knowledge, attitudes, and practices related to CM were completed by HCPs prior to attending the education session and following the 4-month implementation period. Paired t-tests were conducted to determine differences between baseline and follow-up surveys. A total of 31 oncology HCPs completed both baseline and follow-up surveys, with over 3700 patient CM assessment forms being completed during the 4-month study period. At the end of the study, HCPs reported greater CM knowledge (p < 0.001), readiness to support cancer patients' CM decisions (p = 0.002), and willingness to consult with another HCP about CM (p = 0.004). No significant change in HCPs' reported attitudes towards CM, orotherclinical practices related to CM were observed. Implementing a practice guideline, including a CM education session and a standardized assessment form, was found to improve oncology HCPs' self-reported CM knowledge and readiness to answer cancer patients' questions about CM. The findings provide support for future knowledge translation research aimed at standardizing how CM is addressed within cancer care settings.