Low back pain (LBP) or lumbago is a common disorder involving the muscles, nerves, and bones of the back, in between the lower edge of the ribs and the lower fold of the buttocks. Pain can vary from a dull constant ache to a sudden sharp feeling. Low back pain may be classified by duration as acute (pain lasting less than 6 weeks), sub-chronic (6 to 12 weeks), or chronic (more than 12 weeks). The condition may be further classified by the underlying cause as either mechanical, non-mechanical, or referred pain. The symptoms of low back pain usually improve within a few weeks from the time they start, with 40–90% of people recovered by six weeks.

In most episodes of low back pain, a specific underlying cause is not identified or even looked for, with the pain believed to be due to mechanical problems such as muscle or joint strain. If the pain does not go away with conservative treatment or if it is accompanied by "red flags" such as unexplained weight loss, fever, or significant problems with feeling or movement, further testing may be needed to look for a serious underlying problem. In most cases, imaging tools such as X-ray computed tomography are not useful and carry their own risks. Despite this, the use of imaging in low back pain has increased. Some low back pain is caused by damaged intervertebral discs, and the straight leg raise test is useful to identify this cause. In those with chronic pain, the pain processing system may malfunction, causing large amounts of pain in response to non-serious events.

Initial management with non-medication based treatments is recommended. NSAIDs are recommended if these are not sufficiently effective. Normal activity should be continued as much as the pain allows. A number of other options are available for those who do not improve with usual treatment. Opioids may be useful if simple pain medications are not enough, but they are not generally recommended due to side effects. Surgery may be beneficial for those with disc-related chronic pain and disability or spinal stenosis. No clear benefit of surgery has been found for other cases of non-specific low back pain. Low back pain often affects mood, which may be improved by counseling or antidepressants. Additionally, there are many alternative medicine therapies, including the Alexander technique and herbal remedies, but there is not enough evidence to recommend them confidently. The evidence for chiropractic care and spinal manipulation is mixed.

Approximately 9–12% of people (632 million) have LBP at any given point in time, and nearly 25% report having it at some point over any one-month period. About 40% of people have LBP at some point in their lives, with estimates as high as 80% among people in the developed world. Difficulty most often begins between 20 and 40 years of age. Men and women are equally affected. Low back pain is more common among people aged between 40 and 80 years, with the overall number of individuals affected expected to increase as the population ages.