Sir (Rupert) Iain Kay Moncreiffe of that Ilk, Bt., CVO, QC (April 9, 1919 – February 27, 1985) was a British herald and genealogist.

The son of Lieutenant-Commander Gerald Moncreiffe, RN, and Hilda, daughter of the Comte de Miremont, he succeeded his cousin as 11th baronet in 1957. The baronetcy derived from the feudal barony of Moncreiffe, near Perth, Scotland, "of that Ilk" meaning "of that same place", i.e. it is a contraction of "Moncreiffe of Moncreiffe".

Educated at Stowe, Heidelberg, and Christ Church, Oxford, he served in World War II in the Scots Guards, served as attaché at the British embassy in Moscow, and then studied Scots Law at Edinburgh University, where he took a PhD with a thesis on the Scots law of succession to peerages.

A prominent member of the Lyon Court, he held the offices of Falkland Pursuivant (1952), Kintyre Pursuivant (1953), and (from 1961) Albany Herald. He wrote a popular work about the Scottish clans, The Highland Clans (1967), but his interests also extended to Georgian and Byzantine noble genealogies.

He held membership in many London clubs and founded his own club in Edinburgh, called Puffin's, after his first wife, Diana, Countess of Erroll, whom he married in 1946. That marriage was dissolved in 1964 and in 1966 he took as his second wife Hermione Patricia Faulkner, daughter of Lieutenant-Colonel Walter Douglas Faulkner.

He was known as good-natured, witty, knowledgeable, and somewhat eccentric (e.g. he had a trick of moving his false teeth up and down while speaking, which he preferred to play on pretty young girls).