III型分泌系统（英语：Type III secretion system 缩写TTSS或T3SS）是革兰氏阴性菌的一个由多组分蛋白复合体形成的跨膜信道，它通过分泌蛋白，或把这些毒力蛋白直接注入宿主细胞中发挥致病作用。

在病原性细菌中，针状结构被用作传感探针来检测真核生物体的存在和分泌蛋白以帮助细菌感染它们。分泌的效应蛋白是直接从细菌细胞分泌到真核（宿主）细胞，在那里它们施加了许多效果，可有助于病原体存活和逃避免疫反应。

Type three secretion system (often written Type III secretion system and abbreviated TTSS or T3SS, also called Injectisome or Injectosome) is a protein appendage found in several Gram-negative bacteria.

In pathogenic bacteria, the needle-like structure is used as a sensory probe to detect the presence of eukaryotic organisms and secrete proteins that help the bacteria infect them. The secreted effector proteins are secreted directly from the bacterial cell into the eukaryotic (host) cell, where they exert a number of effects that help the pathogen to survive and to escape an immune response.