The globalization of IC manufacturing has increased the likelihood for IP providers to suffer financial and reputational loss from IP piracy. Logic locking prevents IP piracy by corrupting the functionality of an IP unless a correct secret key is inserted. However, existing logic-locking techniques can impose significant area overhead and performance impact (delay and power) on designs. In this work, we propose BISTLock, a logic-locking technique that utilizes built-in self-Test (BIST) to isolate functional inputs when the circuit is locked. We also propose a set of security metrics and use the proposed metrics to quantify BISTLock's security strength for an open-source AES core. Our experimental results demonstrate that BISTLock is easy to implement and introduces an average of 0.74% area and no power or delay overhead across the set of benchmarks used for evaluation.