Background <p>Gestational diabetes mellitus is a multidimensional health problem that may negatively affect lifestyle behaviors and quality of life during pregnancy. Effective management of gestational diabetes mellitus requires self-efficacy in areas such as glycemic monitoring, healthy nutrition, physical activity, and weight control. Motivational interviewing is a patient-centered counseling approach that may support behavior change and health management. This study aimed to determine the effect of motivational interviewing on self-efficacy for health practices and quality of life in pregnant women diagnosed with gestational diabetes mellitus.</p> Methods <p>The study was conducted as a randomized controlled experimental trial. The study data were collected between June 1, 2024, and June 30, 2025, at two hospitals providing services in a city center in southern Türkiye. Participants were randomized into the intervention (n = 30) and control (n = 30) groups. While the pregnant women in the intervention group received four sessions of motivational interviewing, the control group received routine care. A Personal Information Form, the Self-Rated Abilities for Health Practices Scale, and the Short Form-36 Health Survey were used for data collection. To evaluate the effect of motivational interviewing, the changes in the groups over time were compared. The statistical significance level was accepted as <i>p</i> &lt; 0.05.</p> Results <p>Before the intervention, no significant difference was found between the groups in terms of descriptive characteristics or the mean scores of the Self-Rated Abilities for Health Practices Scale and the Short Form-36 Health Survey (<i>p</i> &gt; 0.05). Following motivational interviewing, the intervention group demonstrated significantly higher total Self-Rated Abilities for Health Practices Scale scores than the control group at post-test (92.30 ± 5.33 vs. 72.46 ± 17.95, p &lt; 0.001) and follow-up (93.13 ± 6.21 vs. 71.06 ± 17.21, p &lt; 0.001). The intervention group also had significantly higher scores in the physical functioning (60.16 ± 9.51 vs. 45.33 ± 15.47), vitality (60.50 ± 8.23 vs. 46.33 ± 15.13), mental health (70.53 ± 7.08 vs. 60.66 ± 10.40), social functioning (68.33 ± 15.99 vs. 56.48 ± 19.67), and general health perception (64.66 ± 7.87 vs. 47.32 ± 11.65) domains of the Short Form-36 Health Survey at post-test, with the differences between groups reaching statistical significance (p &lt; 0.05).</p> Conclusion <p>This study demonstrates that motivational interviewing is effective in increasing self-efficacy and quality of life in pregnant women diagnosed with gestational diabetes mellitus. Integrating motivational interviewing into care processes is recommended as an effective approach that can contribute to supporting self-efficacy and quality of life in these pregnant women.</p> Trial registration <p>The study was registered at&#xa0;ClinicalTrials.gov&#xa0;(ID: NCT06243939). First Submitted Date: 15.11.2023.</p>