Low back pain (LBP) is the primary cause of years lived with a disability in the world, but it can be difficult to find a specific structural cause of LBP for the majority of people suffering from it. The focus is now from the passive structures to the neuromuscular regulation of the trunk, and among the various muscles the diaphragm has turned out to be an unexpected but important player. In addition to its well-known function as inspiratory muscle, the diaphragm is involved in postural control and spinal stabilization throughout the day, primarily by controlling and modulating the generation of intra-abdominal pressure (IAP) and the finely-timed co-activation with the transversus abdominis, pelvic floor and lumbar multifidus. This narrative review summarizes current evidence regarding the anatomically dual role of the diaphragm, its mechanism to stiffen and protect the lumbar spine, and imaging and physiological studies that have identified that people with LBP have abnormal diaphragm position, reduced excursion, increased fatigability, and disordered breathing. Epidemiological studies also suggest that breathing and continence disorders are more closely linked to LBP than obesity and physical inactivity. Different assessment methods from dynamic magnetic resonance imaging to bedside ultrasonography are described, and therapeutic options such as diaphragmatic breathing re-education, inspiratory muscle training, dynamic neuromuscular stabilization and manual diaphragm techniques are appraised. The literature as a whole supports the inclusion of breathing assessment and diaphragm-based training in the evaluation and treatment of LBP, recognizes heterogeneity, and calls for larger quality studies.