Out-of-pocket (OOP) healthcare expenditure remains a major barrier to equitable healthcare access and financial protection in Nigeria. The enactment of the National Health Insurance Authority (NHIA) Act 2022 introduced mandatory health insurance as a major policy strategy for advancing universal health coverage and reducing households' exposure to catastrophic healthcare expenditure. This study examined the relationship between health insurance coverage and out-of-pocket healthcare spending among healthcare users in Nigeria. A cross-sectional household survey design was adopted. The original dataset contained 236 respondents drawn from Lagos State, Kano State, Enugu State, Rivers State, and the Federal Capital Territory who had accessed healthcare services within the preceding six months. Following application of the stipulated adult eligibility criterion, 11 respondents younger than 18 years were excluded, yielding a final analytical sample of 225 adults. Health insurance status, duration of enrolment, healthcare-seeking behaviour, total healthcare bill, OOP payment, catastrophic health expenditure, cost-related delay in seeking care, satisfaction with healthcare services, and knowledge of the NHIA were assessed. Descriptive and inferential statistical procedures were employed to compare insured and uninsured respondents. Of the 225 eligible respondents, 142 (63.1%) were insured and 83 (36.9%) were uninsured. Mean OOP expenditure was approximately ₦7,038.03 among insured respondents compared with ₦17,875.90 among uninsured respondents, representing a 60.6% lower mean OOP payment among the insured (p < .001). Catastrophic OOP expenditure and cost-related delayed care were strongly associated with insurance status (Fisher's exact p < .001), while insured respondents reported substantially higher satisfaction (M = 4.17) than uninsured respondents (M = 1.81; p < .001). The study concludes that health insurance coverage is strongly associated with improved financial protection among recent healthcare users. Expansion of effective mandatory coverage, adequate benefit packages, protection of vulnerable populations, improved service quality, and systematic monitoring of financial-protection outcomes are recommended.