The study was conducted to evaluate the effect of inhaler aromatherapy on invasive pain, procedure adherence, vital signs, and saturation during port catheter insertion among patients diagnosed with cancer. The study was conducted in a nonrandomized controlled trial. Sixty patients including 30 patients in the intervention group and 30 patients in the control group, who were subjected with the same local anesthetic protocol, were included in the study. Aromatic mixture prepared by diluting orange, chamomile, and lavender oil in 70 mL distilled water was inhaled by the intervention group during the procedure. The data of the study were collected by using questionnaire, vital follow-up form, and visual analog scale. The patients in the intervention and control groups were similar in terms of sociodemographic and disease characteristics (P > .05). It was determined that inhaler aromatherapy applied to patients in the intervention group decreased pain experienced during the procedure and facilitated the procedure adherence (P < .05); however, it did not affect vital signs and saturation (P > .05). It can be recommended to administer inhaler aromatherapy with pharmacological therapies during catheterization procedure since it decreases invasive pain and facilitates the procedure adherence.