纳米细菌是一种具有细胞壁的微生物。其体直径约为200纳米，是可于正常光学显微镜下成像的对象最小尺寸，大大低于细菌的理论下限。1996年，尚未发现纳米细菌时，这个大小被假定的理由是「现时所知生物需要用到的最小体型」。

有关牠们为生命体的报告是相当具冲击性的。假如这些微小的东西是生存着的，那么就意味着一种与细菌完全不同的崭新生命形式被发现。但最近研究发现纳米细菌只是矿物质与蛋白质形成的晶体，无关生命。其出现可能暗示人体代谢机制出现异常。钙化纳米颗粒 (CNPs) 被用作回避有关这种生命形式的所有难题。虽然纳米生物时常与纳米细菌扯上关系，甚至时而被当作其同义词，可是有关两者的关系暂时并未明朗。

Nanobacterium (NAH-no-bak-TEER-ee-əm, pl. nanobacteria NAH-no-bak-TEER-ee-uh) is the unit or member name of a proposed class of living organisms, specifically cell-walled microorganisms with a size much smaller than the generally accepted lower limit for life (about 200 nm for bacteria, like mycoplasma). Originally based on observed nano-scale structures in geological formations (including one meteorite), the status of nanobacteria has been controversial, with some researchers suggesting they are a new class of living organism capable of incorporating radiolabeled uridine, and others attributing to them a simpler, abiotic nature. One skeptic dubbed them "the cold fusion of microbiology", in reference to a notorious episode of supposed erroneous science. The term "calcifying nanoparticles" (CNPs) has also been used as a conservative name regarding their possible status as a life form.