"We crossed the threshold overnight of a million arrivals in Europe by sea in 2015," said Adrian Edwards, spokesperson of the UN Refugee Agency.
"The vast majority of those attempting this dangerous crossing are in need of international protection," the UN agency said.
There were 1,000,573 arrivals by perilous sea in 2015 with 84 per cent of the arrivals coming from ten countries only.
2015 also registered a whopping 3,735 missing persons and deaths of people trying to cross into Europe on unseaworthy boats and dinghies in a desperate bid to escape the prolonged violent conflicts and economic crises at home.
The top ten nations which account for 90 per cent of the sea arrivals in 2015 were: Syria (49 per cent), Afghanistan (21 per cent), Iraq (8 per cent), Eritrea (4 per cent), Pakistan, Nigeria and Somalia (2 per cent), Sudan, Gambia and Mali (1 per cent).
Of the total sea arrivals in 2015, 58 per cent were men, 25 per cent were children and 17 per cent were women.
The monthly Mediterranean sea arrivals gradually picked up from January onwards and reached a peak in October 2015 which saw more than 2,00,000 arrivals to gradually taper down with the onset of the winter season.
Greece had 8,44,176 arrivals by sea and Italy had 1,52,700 arrivals by sea in 2015.
However, while Syrians, Afghans and Iraqis were the top three nationalities arriving in Greece, Eritreans, Nigerians and Somalis crossed the Ionian sea to reach Italy this year.
The present migrant crisis is considered to be the Europe's worst since World War II.
The number of sea arrivals has increased significantly from 2014 - the year recorded about 2,16,000 arrivals by sea.