The 22-year-old Edinburgh Rugby back-row forward was born on the Indian Ocean island of Mauritius to an English father and a German-born mother.
But his family's globe-trotting past in the British Army have convinced the International Rugby Board he can join new Scotland coach Matt Williams' squad in time for the RBS 6 Nations Championship.
Cross, who had been part of the preparation squad while the IRB considered his case, had already turned down a chance to play for England two years ago.
He said: "It's outstanding news. I have always considered myself Scottish and I'm just very grateful to everyone who has worked on presenting my case to the IRB."
Williams added: "I'm delighted. We have involved Simon fully in our squad preparations for the Six Nations on the basis that we had a very good case to have his eligibility confirmed.
"He has performed well with the squad and he can now concentrate on fighting for his place in the match 22.
"It makes the competition for the back-row very strong."
Cross was born in a British military hospital in Mauritius, his mother, Fiona Chisholm, was born in a British military hospital in Germany and his father, Peter Cross, is an English citizen.
Through the maternal line, the player's grandfather was born in a British military hospital in India, as was his great grandfather.
Only his great, great grandfather was actually born in Scotland.
The IRB Regulations Committee had to be satisfied that the player had a "close, credible and established national link with the country" and in their ruling the committee spoke of a "truly unique set of circumstances."
The committee said in a statement: "The Regulations Committee were strongly persuaded by the weight of the new evidence submitted by the SRU on behalf of the player from the Ministry of Defence of the UK, which acknowledged the status of the hospitals in which the player, his mother and maternal grandfather were born as being British military hospitals.
"The committee accepted that certain rights flow from birth in a British military hospital.
"Further the committee considered that in light of the military service history of the Chisholm family, if the player, his mother and/or grandfather had not been on military service and therefore born in British military hospitals, the strong likelihood is that given the historical circumstances and family lineage at least one of them would have been born in Scotland.
"The committee also took the view that close links to Scotland were further demonstrated by the fact that the player, who was eligible to play for other unions, refrained from representing such other unions.
"In particular he turned down opportunities to play with the England national representative under 21s team in 2002.