Andrew Welsh, who represented Angus at both Holyrood and Westminster, has died at the age of 77.He was first elected to the Commons in October 1974 and represented the South Angus area until he lost the seat in 1979.
He served as the provost for Angus for a period in the 1980s, before returning to Westminster in 1987 after winning the East Angus seat, and remained an MP until 2001.
He left the Commons after being elected to Holyrood in the first Scottish Parliament poll in 1999, and represented Angus there until he retired in 2011.
The First Minister tweeted: "Very sad to hear of the passing of dear friend and colleague, Andrew Welsh, former @theSNP MP & MSP for Angus.
"Andrew was a campaigner for independence long before it had the levels of support enjoyed today. He will be hugely missed."
Deputy First Minister John Swinney also paid tribute on Twitter, saying: "So very sorry to hear of the death of my dear friend and colleague, Andrew Welsh, former Provost, MP and MSP."
He said Mr Welsh had had a "life devoted to serving Angus and Scotland", describing him as a "pioneer of independence and a loyal, true friend and ally".
Former first minister Alex Salmond, who was an SNP MP at the same time as Mr Welsh, said he will be "greatly missed".
Mr Salmond, who now leads the Alba Party, tweeted: "A great sadness to hear of the death of Andrew Welsh, a loyal and talented colleague at Westminster and in the Scots Parliament.
"Andrew provided the backbone of Scottish Nationalist elected representation over four full decades as a parliamentarian and as Provost of Angus."
Scottish Rural Affairs Secretary Mairi Gougeon, who is the current MSP for Angus North and Mearns, tweeted: "So sorry to hear of the passing of Andrew Welsh, former Provost, MP and MSP for Angus.
"A life dedicated to public service and dedicated to the cause of independence, and you just couldn't meet a nicer or more genuine person."