Zero-valent iron (ZVI) supplementation for improving anaerobic digestion (AD) of ammonia-rich swine manure (initial ammonia-N similar to 5000 mg/L) was tested. The addition of 5 g/L ZVI powder apparently accelerated the acidification process to produce more volatile fatty acids (VFAs) and optimized the fermentation type by contributing to a lower system oxidation-reduction potential (ORP) level of -181.7 to-250.0 mV favorable for ethanol-type and butyric-type fermentation during day 14-30, in comparison with that of-164.3 to-216.3 mV in the control group favorable for propionic-type. Overall, ZVI significantly decreased the proportion of propionic acid from 49.8% to 30.9% while increased the proportion of n-butyric acid from 6.8% to 18.7%. Microbial analysis revealed that fast growing and ammonia-tolerant hydrogenotrophic Methanoculleus species were enriched with ZVI, helping achieve a 54.2% higher CH4 yield relative to control. Results from this study demonstrated the potential of ZVI addition to enhance AD of ammonia-rich animal manure.