This study fills a gap with TPACK instrumentation by validating a survey instrument for use specifically with secondary mathematics in-service teachers in the United States using an instrument originally developed in Australia (Handal et al., 2012). A comparable national sample was surveyed in the U.S. to the original Australian instrumentation study. Findings revealed the factor structure of the Australian TPACK instrument differed when used in the U.S. and presents a new validated instrument (TPACK-M-US) for use with secondary mathematics in-service teachers in the U.S. We provide three sources of validity evidence (e.g. Instrument or Test Content, Internal Structure, Response Processes). Appropriate uses and interpretations are discussed in addition to the importance of validation research for educational settings.