ObjectiveTo evaluate an educational intervention based on the Theory of Planned Behavior to increase oncology nurses’ exercise advice behaviors. MethodsA single-group, quasi-experimental repeated measures design study was used with 124 oncology nurses in a Thai cancer institute. The educational intervention included preeducation self-directed activities for 1 hour, a 2-hour group education session, and posteducation outreach contact 1 week later. The outcome measures, including the Theory of Planned Behavior constructs, exercise knowledge, and self-reported exercise prescription behaviors, were collected at three time points: Time 1 (2 weeks preintervention), Time 2 (immediately preintervention), and Time 3 (2 weeks after the intervention). ConclusionResults indicated a statistically significant increase in scores after the intervention for all outcomes, including the Theory of Planned Behavior constructs (overall P value <.001), exercise knowledge (β = 2.99, P < .001), and exercise prescription behaviors: asking (odds ratio [OR] = 12.98, P < .001), assessing (OR = 6.20, P < .001), referring (OR = 4.10, P < .001), and writing exercise advice (OR = 11.52, P < .001). Additionally, verbal counseling behavior was performed by all participants, and therefore, the odds for verbal counseling were not reported. Structural equation modeling analysis demonstrated that attitudes and perceived behavioral control explained 41% of the variance in exercise advice intention. Perceived behavioral control and intention also explained 20% of the variance in self-reported exercise advice behaviors. Implications for Nursing PracticeAttitudinal and perceived behavioral control constructs should be a focus of attention to improve health care professionals’ exercise advice behaviors. More focused research is required to examine whether patients engage in and follow the exercise advice provided by oncology nurses.