Incorporating engineering instruction into the elementary curriculum is not without challenges. Traditionally, researchers investigated using engineering design to promote students learning science concepts. More recently, researchers have conducted qualitative investigations to measure students' learning of engineering concepts after engaging in engineering design. In this study, we extended work on elementary engineering instruction by implementing an integrated engineering and writing unit with 58 third-grade students. Using stratified random assignment based on pre-intervention engineering vocabulary assessment scores, we assigned students to treatment (n = 28) or comparison (n = 30). During a 10-day unit, all students participated in design challenges, emulated the practices of actual engineers, and used writing to support and document their learning, as they designed and authored their own five-page pop-up books. Students in the treatment condition participated in additional writing during 8 of the 10 unit lessons. During this time, they responded to journal prompts related to lesson objectives. At the same time, students in the comparison condition participated in small-group discussions during which they discussed journal prompts orally. We found that all students made statistically significant gains from pre- to posttest on an engineering vocabulary assessment; total words written, number of different engineering concepts used, and depth of understanding of engineering concepts in a written essay response; and number of different engineering concepts used in an oral interview response, regardless of their incoming writing skills and regardless of whether they participated in additional writing or small-group discussion of lesson objectives. This study is the first to quantitatively document the effectiveness of a combined elementary engineering and writing intervention for promoting students' learning of engineering concepts in multiple ways (rote recall, written representation, and oral representation). We argue that literacy, particularly writing, provides an effective and feasible method for incorporating engineering instruction into the elementary curriculum.