Teaching efficacy beliefs have attracted researchers' attention in recent decades because of its close association with and potential impact on the implementation of new ideas and skills in education. In the present study, we have explored the psychometric properties and construct validity of the Turkish adaptation of the Mathematics Teacher Efficacy Belief Instrument developed by Enochs, Smith, & Huinker (2000) for in-service mathematics teachers. The instrument distinguishes between two dimensions of efficacy beliefs for mathematics teachers: personal mathematics teaching efficacy and mathematics teaching outcome expectancy. The sample consisted of 1355 in-service elementary school teachers and middle school mathematics teachers from 368 schools. Exploratory and confirmatory factor analysis revealed a two-factor structure similar to that found in other studies. Also, scores from the two subscales indicated acceptable internal consistency.