Ruairidh Erskine (Scottish Gaelic: Ruaridh Arascain) was a Scottish nationalist, and a member of the landed gentry, inheriting the title "Earl of Mar" (hence he is commonly known as Ruairidh Erskine of Mar). Erskine was born in 1869, in Brighton. He learnt to speak Scottish Gaelic as a child from his nanny who was from the Hebrides.

In 1892, aged 23, he became vice-president of the Scottish Home Rule Association , but he grew to oppose the notion of home rule for Scotland within the United Kingdom and supported all out Scottish independence.

In 1904 he formed the Guth na Bliadhna (Voice of the Year) publication and used it to advocate independence and a coming together of the Gaelic peoples of both Scotland and Ireland to aid each other in a campaign to establish their respective langauges as the official language of their country. He also used the magazine to call for the formation of a political party to campaign for independence.

His activities with the publication brought him into contact with William Gillies, with whom he formed the Scots National League (SNL) in 1920, thus going some way towards the realisation of the formation of a Scottish nationalist political party.

Despite his aristocratic background, Erskine had links with the socialist figure John Maclean who was himself an advocate of an independent (socialist) Scotland. Erskine had at one stage described socialism as "a predatory creed", but by the time of the First World War he was becoming more politically radical and finding sympathy with the cause of figures such as Maclean.

He championed the Easter Rising in Ireland in 1916, and attempted to foster links with the Irish nationalist community by attempting to set up a joint Scottish-Irish Celtic newspaper with Art O'Brien the president of the Irish Self Determination League .

Erskine attempted to get independent representation for Scotland at the Paris Peace Conference at the end of the First World War. In this he was ultimately unsuccesful, but it did attract the support of figures such as James Maxton, a prominent figure in the Independent Labour Party at the time.

Erskine and Gillies led the SNL into joining with other groups to form the National Party of Scotland (NPS) in 1928. The NPS was quite different in outlook to the SNL had been and many SNL members left the NPS due to this factor, including Erskine. After this, Erskine was to play little role in politics.

Although, Erskine has been accused of being reactionary, it is noteworthy that some of his ideas have gained a lot of popular credibility over time, e.g. Scottish indepedence, land reform (through the land league), and even the Scottish Gaelic revival. His pan-Celticism has received some attention from the Celtic Congress and Celtic League

He died in 1960.