<p>The USA established and operated numerous Indian boarding schools nationwide from the late 1800s through the twenty-first century. These government institutions were part of an assimilation and Americanization policy for Indigenous Peoples through education. Previous research has documented how all aspects of the boarding schools’ curricula and practices, including mathematics instruction, were designed to assimilate Indigenous youth to Anglo-American standards and eradicate their identities, languages, and cultural knowledge. However, no study has examined the mathematical concepts taught in these colonizing institutions and how the mathematical knowledge made available in Indian schools compares to other curricula. In this current study, we examined arithmetic teaching materials found in an archive of documents kept by Estelle Reel, the superintendent of Indian schools from 1898 to 1910, who oversaw the training of teachers working in the boarding schools. In our analysis, we focused on classifying arithmetic word problems used in Grades K–8 according to the concepts and complexity of the arithmetic and compared frequency and characteristics across grades. In addition to the four basic arithmetic operations, there is an emphasis on unit conversion, geometry, and exponential equations with increasing complexity with grade level. The distribution of additive problems is significantly different from seven out of eight textbooks published in the 1980s and the Common Core era. We discuss our findings in light of the literature on the history of school mathematics and Indian boarding schools in the USA.</p>