Greenmail or greenmailing is a corporate acquisition strategem for generating large amounts of money from the attempted hostile takeover of large, often undervalued or inefficient companies. It proved lucrative for investors such as T. Boone Pickens and Sir James Goldsmith during the 1980s.

Greenmailing is a variant of the corporate raid strategy of taking over an undervalued company, dismembering it, and selling off its valuable pieces for a profit. Once having secured a large share of a target company, instead of completing the hostile takeover the greenmailer offers instead to end the threat to the victim company by selling his share back to it—at a vastly inflated price, however. To preserve its own existence, the victim will often pay the exorbitant premium demanded. Goldsmith, for example, made $90 million from the Goodyear Tire and Rubber Company in the 1980s in this manner.

Changes in the details of corporate ownership structure and in the investment markets generally have made greenmail far less common since the early 1990s.