Thomas Boothby Parkyns, 1st Baron Rancliffe (24 July 1755 – 17 November 1800) was an English soldier, Member of Parliament and Irish peer.
He was the eldest son of Sir Thomas Parkyns, 3rd Baronet (1728–1806) of Bunny Hall, Nottinghamshire and was educated at Queen's College, Cambridge.
As an officer in the British Army he was, from 1783 to 1790, equerry to HRH Prince Henry, Duke of Cumberland and Strathearn, the third son of the Frederick, Prince of Wales. A close friend of the Prince of Wales, he was made Colonel of the Prince of Wales Fencibles in 1794. After Prince Henry's death in 1790, Parkyns took over the lease of Rookley Manor, Hampshire until his own death from oedema in 1800, whereby he predeceased his father.
He was elected MP for in 1784, sitting until 1790 and then as MP for Leicester in 1790, sitting until 1800.
In 1787 he was elected a Fellow of the Royal Society as "a Gentleman well versed in various branches of Science" In 1795 he was made an Irish peer as Baron Rancliffe.
He had married Elizabeth Anne, the daughter of Sir William James, MP (who died at the wedding) and had nine children. He was succeeded by his son George.