This paper examines how a concentrated tenant base affects the operating performance and market valuations of US REITs. We observe that REITs adopting a concentrated tenant base present higher corporate cash flows and lower expenses. However, we identify a concentration discount effect that REITs with a more concentrated tenant base experience lower market valuations. We argue that this concentration discount is a result of the trade-offs between the impacts of the tenant base on the operating performance, risk levels and growth potentials. We find that a concentrated tenant base is associated with higher liquidity risk and lower dividend growth, resulting in an inflated discount factor. Our findings are not subject to sub-samples of focused or diversified REITs and stay robust after correcting for the selection bias as well as controlling for the lease structure, tenant quality and anchor tenant effect