Magnetic resonance elastography (MRE), an advanced magnetic resonance–based imaging technique, and acoustic radiation force impulse (ARFI), an ultrasound‐based imaging technique, are accurate for diagnosing nonalcoholic fatty liver disease (NAFLD) fibrosis. However, no head‐to‐head comparisons between MRE and ARFI for diagnosing NAFLD fibrosis have been performed. We compared MRE versus ARFI head‐to‐head for diagnosing fibrosis in well‐characterized patients with biopsy‐proven NAFLD. This cross‐sectional analysis of a prospective cohort involved 125 patients (54.4% female) who underwent MRE, ARFI, and contemporaneous liver biopsies scored using the Nonalcoholic Steatohepatitis Clinical Research Network histological scoring system. The performances of MRE versus ARFI for diagnosing fibrosis were evaluated using area under the receiver operating characteristic curves (AUROCs). The mean (± standard deviation) age and body mass index were 48.9 (±15.4) years and 31.8 (±7.0) kg/m², respectively. For diagnosing any fibrosis (≥ stage 1), the MRE AUROC was 0.799 (95% confidence interval [CI] 0.723‐0.875), significantly (P = 0.012) higher than the ARFI AUROC of 0.664 (95% CI 0.568‐0.760). In stratified analysis by presence or absence of obesity, MRE was superior to ARFI for diagnosing any fibrosis in obese patients (P < 0.001) but not in nonobese patients (P = 0.722). The MRE AUROCs for diagnosing ≥stages 2, 3, and 4 fibrosis were 0.885 (95% CI 0.816‐0.953), 0.934 (95% CI 0.863‐1.000), and 0.882 (95% CI 0.729‐1.000); and the ARFI AUROCs were 0.848 (95% CI 0.776‐0.921), 0.896 (95% CI 0.824‐0.968), and 0.862 (95% CI 0.721‐1.000). MRE had higher AUROCs than ARFI for discriminating dichotomized fibrosis stages at all dichotomization cutoff points, but the AUROC differences decreased as the cutoff points (fibrosis stages) increased. Conclusion: MRE is more accurate than ARFI for diagnosing any fibrosis in NAFLD patients, especially those who are obese. (Hepatology 2016;63:453–461)