We explore a policy-induced change in borrower ability to shop for mortgages to investigate whether market competitiveness affects mortgage interest rates. Our paper exploits a discontinuity in the competitive landscape introduced by the Home Affordable Refinancing Program (HARP). Under HARP, lenders that currently service loans eligible for refinancing enjoyed substantial advantages over their potential competitors. Using a fuzzy regression discontinuity design, we show a jump in mortgage interest rates precisely at the HARP eligibility threshold. Our results suggest that limiting competition raised interest rates on 30-year fixed-rate mortgages by 15 to 20 basis points, translating into higher lender profits. The results are distinct from documented effects of consolidation and capacity reduction in mortgage lending and are robust to a number of sample restrictions and estimation choices. We interpret our findings as evidence that increases in pricing power lead to higher interest rates in mortgage markets.