Agrobacterium tumefaciens is a species of bacteria that causes tumors (commonly known as 'galls' or 'crown galls') in dicots. This bacterium causes crown gall by inserting a small segment of DNA (known as the T-DNA, for 'transfer DNA') into the plant cell, which is incorporated at a semi-random location into the plant genome.

A. tumefaciens is commonly used as a vector to introduce DNA carried on a plasmid into a recipient organism. The vector is 'disarmed' since it lacks the genes which encode the tumorigenic functions of A. tumefaciens. This type of vector has been used frequently for introducing new genes into plants without causing biosafety concerns: the procedures used generally ensure that no Agrobacterium remains in the regenerated transgenic plants.