噬菌体（英语：bacteriophage）是病毒的一种，其特别之处是专以细菌为宿主，较为熟知的噬菌体是以大肠杆菌为寄主的T2噬菌体。

跟别的病毒一样，噬菌体只是一团由蛋白质外壳包裹的遗传物质，大部分噬菌体还长有「尾巴」，用来将遗传物质注入宿主体内。超过95%已知的噬菌体以双螺旋结构的DNA为遗传物质，长度由5,000个碱基对到5,000,000个碱基对不等；余下的5%以RNA为遗传物质。正是通过对噬菌体的研究，科学家证实基因以DNA为载体。（见赫希-蔡斯实验）整个噬菌体的长度由20纳米到200纳米不等。它们的基因组可含有少至四个、多至数百个基因。在注射其基因组进入细胞质后，噬菌体在细菌内复制。噬菌体是在生物圈中最常见的和多样化的实体。

A bacteriophage /ˈbækˈtɪər.i.oʊˌfeɪdʒ/ (informally, phage /ˈfeɪdʒ/) is a virus that infects and replicates within a bacterium. The term is derived from "bacteria" and the Greek:φαγεῖν (phagein), "to devour". Bacteriophages are composed of proteins that encapsulate a DNA or RNA genome, and may have relatively simple or elaborate structures. Their genomes may encode as few as four genes, and as many as hundreds of genes. Phages replicate within the bacterium following the injection of their genome into its cytoplasm. Bacteriophages are among the most common and diverse entities in the biosphere.