English abstract Context: Cardiac catheterization is an invasive diagnostic and therapeutic procedure that causes patients to experience excruciating pain. Breathing slowly and deeply as a non-pharmacological approach; results in relaxation that improves oxygenation, This study aimed to evaluate the effect of deep breathing exercises on pain after cardiac catheterization in adults. Methods: The experimental design used in this study is randomized controlled single-blind. The sample included 60 patients selected randomly from the cardiac catheter unit of Alahrar Hospital and divided equally into either a control group or intervention group (n=30 each). Deep breathing exercises were performed four times, each for ten minutes, by the intervention group. Both groups had their pain levels measured at two, four, and six hours after the intervention. Results: In test one, the mean pain level score was (31.43±2.49) in the intervention group and 29.07±2.33 in the control group with a statistically significant difference at (p<0.001). However, at test three, the mean score level decreased to 23.83±1.95 in the intervention group while increasing to 31.47±2.49 in the control group with a highly statistically significant difference (P<0.001). Conclusion: The study found that breathing slowly and deeply after cardiac catheterization was effective and safe in reducing pain, especially if performed routinely and effectively. Relevance to practice: Breathing exercises could be applied in patients with invasive and non-invasive procedures to alleviate their pain. Deep breathing exercises could be a lifestyle. Key words: Deep Breathing Exercises, Cardiac Catheter, Pain, and Single-Blind Method.