Mary Montagu, Duchess of Montagu (c.1711/1712 – 2 May 1775), known as Countess of Cardigan between 1730 and 1749, was the wife of George Brudenell, 4th Earl of Cardigan.

In 1766, her husband the earl was made a duke, reviving the titles that had become extinct as a result of his father-in-law's death without a male heir.

Lady Mary married the earl on 7 July 1730 at St Giles-in-the-Fields.

The duchess died at the age of about 63, and was buried at St Edmund's Church, Warkton, Northamptonshire.

[2] Her memorial was sculpted by Peter Mathias van Gelder to a design by Robert Adam, and was completed in 1781.