Sir Jacob Garrard Downing, 4th Baronet MP (c. 1717 – 6 February 1764) was an English baronet and politician. He was the grandson of Sir George Downing, 1st Baronet, namesake of Downing Street; and cousin and heir of Sir George Downing, 3rd Baronet, whose estate would eventually establish Downing College, Cambridge despite illegal attempts by Jacob and his widow to prevent the university from receiving the estate.
Downing was the oldest son of Charles Downing of Bury St Edmunds, Suffolk, and Sarah Garrard. He was named after his maternal grandfather, Jacob Garrad, who was son and heir of Sir Thomas Garrad, 2nd Baronet of Langford, but who died before his father and thus did not succeed as the third baronet. Jacob Downing had an older sister, Sarah, who died as a child before he was born. His father was comptroller of customs in Salem, Massachusetts, and became immensely wealthy, and he inherited his father's wealth in 1740.
Downing was a Member of Parliament for Dunwich from 1741 to 1747, from 1749 to 1761 and from 1763 to 1764. He was closely associated with Lord Hardwicke.
In 1749, Downing inherited the baronetcy and estate from his cousin Sir George Downing, 3rd Baronet. The next year, on 17 May 1750, Downing married Margaret Price, daughter of Rev. Price, curate at Barrington, Gloucestershire. They had no children.
Downing devoted himself to restoring his late cousin's lands, which were left in his hands in a poor state. Near the end of his life, Sir George Downing had failed to find tenants for vacant farms, and was losing £1,500 a year (equivalent to £214,660 in 2015) through his mismanagement.