Embodiments relate to dynamic enablement of multithreading. According to an aspect, a computer system includes a configuration with a core configurable between a single thread (ST) mode and a multithreading (MT) mode. The ST mode addresses a primary thread, and the MT mode addresses the primary thread and one or more secondary threads on shared resources of the core. The computer system also includes a multithreading facility configured to control the configuration to perform a method. The method includes executing in the primary thread in the ST mode, an MT mode setting instruction. A number of threads requested is obtained from a location specified by the MT mode setting instruction. Based on determining that the number of threads requested indicates multiple threads, the MT mode is enabled to execute the multiple threads including the primary thread and the one or more secondary threads.