The widespread use of online reviews in the automotive spare-parts market, combined with the difficulty of assessing part authenticity, quality, and technical compatibility, has made review credibility an important factor in reducing buyer uncertainty. This study examined the relationship between perceived online review credibility, acquired product knowledge, and spare-parts purchase decision-making, and evaluated the role of customer knowledge in this relationship. The study was applied in purpose and quantitative, cross-sectional, and correlational in method. The sample comprised 400 consumers aged 18 years or older in Tehran, Mashhad, Isfahan, Shiraz, and Tabriz who had participated in purchasing a spare part during the previous 12 months and had read online reviews before purchase. Data were collected using a 20-item, five-point questionnaire measuring prior knowledge, review credibility, acquired knowledge, and purchase decision. Cronbach's alpha coefficients ranged from .762 to .832. Analyses were conducted in SPSS 26 using descriptive statistics, exploratory factor analysis, Pearson correlation, and hierarchical regression. Review credibility positively predicted acquired knowledge (β = .456, p < .001; R² = .300). The total effect of credibility on purchase decision was significant (β = .301, p < .001) and remained significant after acquired knowledge was entered (β = .129, p = .011). Acquired knowledge also predicted purchase decision (β = .376, p < .001), and the final model explained 27.5% of the variance. Because a bootstrap confidence interval for the indirect effect was unavailable, no definitive conclusion was drawn regarding mediation. The findings emphasize the importance of credible reviews and verifiable technical information in spare-parts sales platforms.