Teach For America (TFA) is an important but controversial source of teachers for hard-to-staff subjects in high-poverty U.S. schools. We present findings from the first large-scale experimental study of secondary math teachers from TFA. We find that TFA teachers are more effective than other math teachers in the same schools, increasing student math achievement by 0.07 standard deviations over one school year. Addressing concerns about the fact that TFA requires only a two-year commitment, we find that TFA teachers in their first two years of teaching are more effective than more experienced non-TFA teachers in the same schools.