Background: Prediabetes mellitus (pre-DM) is an important predictive indicator of Type 2 diabetes. A person with pre-DM is eight times more likely to develop diabetes than a person without pre-DM. Prior research suggests that proactive interventions may delay the progression of this disease and reduce the rate of disease development. Purpose: The purposes of this preliminary study were to develop a multitheory-driven lifestyle intervention protocol for adults with pre-DM and to evaluate its feasibility and impacts on knowledge regarding pre-DM, dietary behaviors, and physical activity (primary outcomes) as well as to describe the disease progression indicators (secondary outcomes). Methods: A single-group, longitudinal study design was used. Thirty-nine participants were included in the analysis. A generalized estimating equation model was used to determine the trends in changes in the outcomes. All of the participants underwent testing at baseline (T0) and at 3 (T1), 6 (T2), and 12 (T3) months after the 4-week lifestyle intervention. Results: There were significantly increasing trends for each study parameter (Pre-DM Knowledge Assessment Form-12, p G .01; Dietary Behavior Scale, p G .01) and significantly positive changes in body weight (p G .01), body mass index (p G .01), fasting glucose level (p G .01), and glycated hemoglobin level (p G .01) over the 12-month study period. Conclusions/Implications for Practice: This study supports the feasibility of the developed multitheory-driven lifestyle intervention protocol and suggests that its application may improve the effectiveness of diabetes prevention programs in clinical settings. Further randomized controlled trials are needed.