This exploratory study evaluated the effects of self-regulated strategy development (SRSD) on students' written expression when asked to solve a word problem and explain reasoning. A secondary question evaluated potential for differential effects based on determination of having mathematics difficulty (MD). We implemented a quasi-experimental design with 163 students with and without MD in grade 4. Results indicated that students in the treatment group (n = 83) significantly outperformed students in the comparison group (n = 80) on measures of mathematical writing involving subtraction and division word problems. For the subtraction problems, students in the treatment group improved their overall written expression of mathematics scores (g = 0.688), yielding a medium effect. The intervention differentially benefited students without MD. Our findings call for greater integration of systematic writing opportunities in mathematics and highlight the importance of explicitly teaching students how to approach the written expression of their mathematical reasoning.