Development corridors are spreading worldwide, creating both opportunities for economic growth and environmental risks. Strategic environmental assessment (SEA) offers a potential framework for planning more sustainable corridors. However, research on SEA application and effectiveness in corridor planning is scarce, particularly in Africa. This case study assessed the procedural effectiveness of two SEAs for the master plan of the Northern Economic Corridor in East Africa. Data were collected from the SEA reports and analyzed using document analysis guided by best practice, strategic criteria derived from the literature. Results show the SEAs had limited procedural effectiveness, focusing on impact prediction and mitigation rather than developing a sustainability strategy. The case study offers valuable insights into the challenges and opportunities of implementing SEA in complex development corridors. The paper makes policy recommendations for more strategic assessments, towards the establishment of more robust SEA systems and more successful corridor investments in Africa and beyond.