We propose that auditor-provided tax services (tax NAS) improve internal control quality by accelerating audit firm awareness of transactions material to the financial statements. Using data from 2004 to 2012, we find robust evidence that companies purchasing tax NAS are significantly less likely to disclose a material weakness and that this result is not due to auditor independence impairment. A one standard-deviation increase in tax NAS is associated with approximately a 13 percent decrease in the rate of material weaknesses relative to the base rate. These results are robust to tests addressing endogeneity concerns. Additional cross-sectional analyses reveal expected increased effects of tax NAS on internal control quality (1) after significant operational changes that require changes to the internal control structure, and (2) earlier in the relationship with the financial statement audit firm, when there are fewer established lines of communication between the audit team and client. This paper contributes to the knowledge spillover literature by identifying a mechanism through which tax NAS improve overall financial reporting quality.