Digital transformation in tax administration can influence taxpayer compliance not only by strengthening monitoring capacity, but also by reshaping service quality, administrative burden, perceptions of fairness, and institutional trust. This study examines the tax compliance behavior of Iranian small and medium-sized enterprises (SMEs) within the Taxpayer System, with particular emphasis on Digital Tax Service Quality in the Taxpayer System, Perceived Administrative Burden Arising from the Taxpayer System, Perceived Tax Fairness, and Trust in the Iranian National Tax Administration. The study is applied in purpose and adopts a quantitative, cross-sectional, survey-based design. Data collected from 309 questionnaires were analyzed using partial least squares structural equation modeling (PLS-SEM). The results show that Digital Tax Service Quality reduces Perceived Administrative Burden Arising from the Taxpayer System and has a positive direct effect on Tax Compliance behavior. Perceived Tax Fairness positively affects both Trust in the Iranian National Tax Administration and Tax Compliance behavior, whereas Perceived Administrative Burden Arising from the Taxpayer System negatively affects both trust and compliance behavior. Trust in the Iranian National Tax Administration exhibits the strongest positive direct effect on Tax Compliance behavior, with a standardized coefficient of 0.421. The indirect effects of service quality through administrative burden, perceived fairness through trust, and the sequential pathway linking service quality, administrative burden, trust, and compliance are all statistically significant. The model explains 57.4% of the variance in Tax Compliance behavior. Among the control variables, only financial/accounting/tax work experience has a positive and significant effect. The findings underscore the importance of simultaneously improving digital tax service quality, reducing administrative burden, strengthening perceived fairness, and fostering institutional trust.