Postoperative pain is considered a major, complex and multidimensional problem that affects the clinical and functional outcomes of patients and may contribute to increased postoperative complications. The goal of this study was to determine the prevalence of inadequate pain control and to investigate the factors associated with inadequate pain control among postoperative cancer patients. A descriptive correlational, cross-sectional design was used. The study was conducted at four adult inpatient oncology departments at King Hussein Cancer Center, a nongovernmental, nonprofit, comprehensive hospital for treating cancer patients in Amman, Jordan. The convenience sample of 800 cancer patients selected comprised postoperative patients diagnosed with cancer and aged ≥18 years who were willing to participate and able to use the numeric rating scale. About 32.9% of patients had pain scores higher than 4/10 at rest, and 56.4% of patients had pain scores higher than 4/10 on movement. Data revealed that patients aged between 18 and 63 years (odds ratio [OR] = 0.196,p< .0005, and OR = 0.245,p< .0005) and chronic user patients (OR = 28.029,p< .0005, and OR = 10.332,p< .0005) had increased odds of poor pain control at rest and on movement, respectively. Administration of preemptive medications and of fentanyl and bupivacaine during the postoperative period was significantly associated with decreased odds of poor pain control. The intravenous route was associated with increased odds of poor pain control at rest and on movement (OR = 2.279,p= .016, and OR = 5.393,p= .012) compared with other routes, including combinations of the intravenous and oral or epidural route. Chronic use of pain medications and older age were predictors of inadequate pain control postoperatively. Administration of preemptive medications and of combinations of fentanyl and bupivacaine via the epidural route was associated with better pain control.