An in-system memory testing method includes transparently selecting and "stealing" a portion of memory from the memory system for running memory tests, then running one or more of the numerous known memory system tests on the selected portion of memory, and then inserting the selected, and now tested, portion of memory back into the system for normal application use. The disclosed in-system memory testing method is capable of testing system memory in both offline and online environments, without imposing any additional hardware requirements or significantly affecting system performance. The disclosed in-system memory testing method is compatible with any conventional prior art functional test algorithm for in-system memory testing and can be performed under real life system environmental conditions. Therefore, the disclosed in-system memory testing method complements other test techniques like BIST/POST that are conventionally used only at the time of system boot up.