Genetically modified mammals are mammals that have been genetically engineered. They are an important category of genetically modified organisms. The majority of research involving genetically modified mammals involves mice with attempts to produce knockout animals in other mammalian species limited by the inability to derive and stably culture embryonic stem cells.

The majority of genetically modified mammals are used in research to investigate changes in phenotype when specific genes are altered. This can be used to discover the function of an unknown gene, any genetic interactions that occur or where the gene is expressed. Genetic modification can also produce mammals that are susceptible to certain compounds or stresses for testing in biomedical research. Some genetically modified mammals are used as models of human diseases and potential treatments and cures can first be tested on them. Other mammals have been engineered with the aim of potentially increasing their use to medicine and industry. These possibilities include pigs expressing human antigens aiming to increasing the success of xenotransplantation to lactating mammals expressing useful proteins in their milk.

Genetically modified mice are often used to study cellular and tissue-specific responses to disease (cf knockout mouse). This is possible since mice can be created with the same mutations that occur in human genetic disorders, the production of the human disease in these mice then allows treatments to be tested.

The oncomouse is a type of laboratory mouse that has been genetically modified developed by Philip Leder and Timothy A. Stewart of Harvard University to carry a specific gene called an activated oncogene.

Metabolic supermice are the creation of a team of American scientists led by Richard Hanson, professor of biochemistry at Case Western Reserve University at Cleveland, Ohio. The aim of the research was to gain a greater understanding of the PEPCK-C enzyme, which is present mainly in the liver and kidneys.

In 4 March, 2025, Colossal biosciences announced they had successfully bred a "woolly mouse" as part of their ongoing efforts to produce the first genetically modified elephant calf with mammoth characteristics by the end of 2028.

A knockout rat is a rat with a single gene disruption used for academic and pharmaceutical research.

BioSteel is a trademark name for a high-strength based fiber material made of the recombinant spider silk-like protein extracted from the milk of transgenic goats, made by Nexia Biotechnologies. Prior to its bankruptcy, the company successfully generated distinct lines of goats that produced recombinant versions of either the MaSpI or MaSpII dragline silk proteins, respectively, in their milk.