Pregnancy is a sensitive period in a woman's life. During this time, pregnant mothers experience stress due to various factors, such as physiological, psychological, and social adaptations. This can lead to high levels of stress, especially among high-risk mothers. Guided imagery is an effective relaxation technique that can reduce stress, providing relaxation to both the mind and body. It alleviates stress and mitigates negative pregnancy outcomes. The aim of this study is to assess the effectiveness of guided imagery on stress among high-risk pregnant mothers in selected hospitals in Kanyakumari District, Tamil Nadu, India. A quantitative approach with a quasi-experimental pretest-posttest control group design was adopted. Sixty high-risk pregnant mothers, 30 in the experimental group and 30 in the control group, were selected using a non-probability purposive sampling technique. The Perceived Prenatal Maternal Stress Scale was used to assess the stress levels of high-risk mothers. In the experimental group, the pretest mean stress score was 31.63± 8.65, and the posttest mean stress score was 13.16 ± 1.23. The mean difference was 18.46, and the 't' value was 11.94, which was statistically significant at the 0.05 level. The study revealed that guided imagery was effective in reducing stress among high-risk pregnant mothers.