We investigate the effect of ETF ownership on stock market anomalies and market efficiency. We find that low ETF ownership stocks exhibit higher returns, greater Sharpe ratios, and highly significant alphas in comparison to high ETF ownership stocks. We show that high ETF ownership stocks demonstrate more pronounced information flows than low ETF ownership stocks which reduces their mispricing as they are more informationally efficient. We find similar results when we match the two groups based on size, volume, book-to-market and momentum. Our results are robust to different matching methods and to a wide array of controls in Fama-MacBeth regressions. Using Russell index reconstitution, we find causal evidence that ETF ownership attenuates anomaly returns.