Our statisticians developed and tested a model. We examined the effects of motivation on mathematics performance. We tested the model using data from the Third International Mathematics and Science Study (1999) on the mathematics performance of eighth-grade students in the USA. Structural equation modeling and multilevel path modeling analyses were conducted. In conclusion, intrinsic motivation positively influenced mathematics performance, whereas external regulation negatively influenced mathematics performance. Additionally, a positive mathematics self-concept significantly affected mathematics performance both directly and indirectly through the mediator of intrinsic motivation. Finally, autonomy support in the classroom significantly affected mathematics performance both directly and indirectly through the mediator of mathematics self-concept.