A germline mutation, or germinal mutation, is any detectable variation within germ cells (cells that, when fully developed, become sperm and ovum).

Mutations in these cells are the only mutations that can be passed on to offspring, when either a mutated sperm or oocyte come together to form a zygote.

After this fertilization event occurs, germ cells divide rapidly to produce all of the cells in the body, causing this mutation to be present in every somatic and germline cell in the offspring; this is also known as a constitutional mutation.

Germline mutation is distinct from somatic mutation.