Students perceived linguistic competence and textbook linguistic complexity are related to their engagement in science learning, yet the mechanisms linking linguistic factors to achievement remain insufficiently examined, particularly in multilingual contexts. This study investigated the effects of perceived linguistic competence and textbook linguistic complexity on science achievement through engagement profiles among 7th-grade students in Ethiopia. A correlational research design was employed. Data were collected from 500 students for the survey and from 210 students for the linguistic complexity analysis in selected primary schools in Bahir Dar city using multistage, stratified, and content sampling techniques. Perceived linguistic competence across phonological, lexical, and semantic dimensions, perceived textbook linguistic complexity, and engagement profiles across cognitive, behavioural, emotional, and agentic dimensions were assessed using validated instruments. Linguistic complexity of selected reading texts in the 7th-grade General Science textbook was analyzed through a triangulated approach incorporating a Linguistic Complexity Judgment Task, lexical density measures, lexical sophistication, and lexical diversity indices. Science achievement was measured using official second semester average scores. Structural Equation Modeling (SEM) was used to test direct and indirect effects. The findings showed that the reading texts had high lexical density, sophisticated vocabulary, and abstract meanings, indicating a high level of linguistic complexity in the general science textbook. The SEM results also showed that perceived linguistic competence and textbook linguistic complexity had both direct and indirect effects on science achievement through engagement profiles. Engagement profiles partially mediated the effects of perceived linguistic competence and textbook linguistic complexity on science achievement, with cognitive and agentic engagement showing stronger mediating roles. The findings emphasized that linguistic perceptions and text demands influence achievement during a key stage of schooling. It is recommended that science instruction and textbooks be aligned with students’ linguistic competence and designed to enhance engagement in order to improve science achievement