下背痛（英语：low back pain，有时简称为LBP或lunbago，英语发音：/lʌmˈbeɪɡoʊ/）也称「腰痛」，是常见背部肌肉骨骼伤病，对许多民众生活构成影响。根据疼痛持续时长，下背痛可分为急性（小于6周）、亚慢性（6至12周）与慢性（大于12周）。依照其成因，则可进一步分为机械性疼痛、非机械性疼痛和转移痛。

绝大多数的下背痛无法找到或确认病源，只能推测这类下背痛是因物理伤害所导致，如扭伤或挫伤。如下背痛经保守疗法仍无法止痛，或是伴随有一些「危险征象」，如不明原因的体重减轻、发烧，抑或在行动或感觉上有明显异状，则需进一步检查是否有较严重的潜在问题。X射线电脑断层扫描之类影像检查在多数情况下帮助不大，而且检查本身也具风险，但仍是常见的下背痛检查手段。部分下背痛源自椎间盘损伤，举腿试验是这种损伤的有效辨别方式。对慢性下背痛而言，有时疼痛处理系统会失常，造成即便只受到轻度刺激，也会有剧烈的疼痛反应。

Low back pain (LBP) is a common disorder involving the muscles, nerves, and bones of the back. 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 completely better by six weeks.