It said that as of Monday, taking into account the latest updates, there had been 1,005,504 "irregular arrivals" into Europe in 2015.
The figures show that the vast majority -- 971,289 -- have come by sea over the Mediterranean. Another 34,215 have crossed from Turkey into Bulgaria and Greece by land.
Among those traveling by sea, 3,695 are known to have drowned or remain missing as they attempted to cross the sea on unseaworthy boats, according to IOM figures. That's a rate of more than 10 deaths each day this year.
More than 4 million Syrians have fled the conflict, creating the worst refugee crisis seen in 25 years, according to the United Nations.
Afghans accounted for 20% of the migrant flow, and Iraqis 7%.
A 'manageable' crisis
The startling human tide has presented European leaders, already grappling with the Eurozone
debt crisis, with a fresh challenge -- one that has created political rifts and thrown the European goal of border-free travel into question.
IOM Director General William Lacy Swing said the numbers were significant, but not unmanageable.
"The numbers are important, but there's also a recognition that they're going into a population area of 550 million," he told CNN.
"If there were not a crisis of solidarity and leadership within the European Union, whereby others would follow the very important, courageous and visionary leadership of (German) Chancellor (Angela) Merkel and open their doors, then dispersed among 28 countries, it would have been much more manageable."
Germany, the most economically powerful country in the European Union, has led the way in extending a welcome to migrants, becoming the destination of choice for many entering Europe.
The country is set to take in more than a million asylum seekers this year, considerably more than any other country in the European Union
, and has set aside more than $6 billion to help feed and house the new arrivals.
But where Merkel has opened doors, other European leaders have put up fences.
Swing contrasted Europe's migrant burden with that of Syria's neighbors, who were accommodating most of the 4 million refugees from the conflict.
"Turkey is now the largest refugee-hosting country in the world, with close to 2.5 million," he said.
"Lebanon, with a population of less than 5 million, is hosting more than a million, and water-poor Jordan is giving 10 million liters of water every day to the million in their refugee camps."
"You cannot have unbroken simultaneous conflict from the western bulge of Africa to the Himalayas without expecting that a lot of people will be heading north, and obviously the resolution of the Syrian conflict is key to everything here," he said.
The numbers of migrants into Europe have exploded in 2015 as an unprecedented surge of people have fled wars, persecution and poverty in the Middle East and Africa.
More than 970,000 migrants have entered Europe by crossing the Mediterranean so far this year, dwarfing the 219,000 who made the same crossing in 2014, according to the United Nations
More people -- 221,000 -- crossed the Mediterranean in October of 2015, the busiest month for migration into Europe, than in the whole of 2014.
The Mediterranean, where nearly 3,700 people died this year, has become "the deadliest route for migrants on our planet," Swing has said.
At least 11 more, including three children, died Tuesday when their boat capsized while making the perilous journey from Turkey to Greece, Turkey's semiofficial Anadolu Agency reported.
By contrast, there have been 6,029 deaths between 1998 and 2013 along the second-deadliest border: the one between the United States and Mexico, the International Organization for Migration said.
Deaths are also commonplace along land routes into Europe. In August, Austrian authorities found the bodies of 71 migrants
, believed to be from Syria
, who had suffocated in a truck abandoned along a highway.
More than 80% through Greece
The overwhelming majority of migrants -- 821,008, or 81.6% -- arrived in Europe in Greece, the IOM said. The second-highest number of arrivals -- 150,317 -- were in Italy, with the remainder in Bulgaria, Spain, Cyprus and Malta, in that order.
According to an IOM statement released this month, the top five nationalities arriving in Greece were from Syria, Afghanistan, Iraq, Pakistan and Albania.
The top five nationalities arriving in Italy were Eritrean, Nigerian, Somalian, Sudanese and Syrian, the IOM said, citing figures from the Italian Interior Ministry.
Eritreans are running from a life of repression and abject poverty, fleeing "one of the poorest countries in the world and a closed and highly securitized state under an authoritarian government," according to a report by the Regional Mixed Migration Secretariat in Nairobi last year
European leaders have hashed out a plan to resettle 160,000 refugees. But a massive gap remains between what they have pledged to do and what remains to be done.
And public sentiment toward the migrants remains mixed across the continent.
A photograph of a drowned 3-year-old Syrian Kurdish boy on a Turkish beach triggered an international wave of public sympathy for migrants.
But subsequent events, such as the revelation that one of the Paris attackers entered Europe alongside Syrian migrants landing in Greece, have helped cool goodwill and have boosted the fortunes of far-right political movements.
France's anti-immigration National Front party won an unprecedented 27% in a nationwide vote
in regional elections this month.
U.N. High Commissioner for Refugees António Guterres said that in the face of rising anti-migrant sentiment, it was "important to recognize the positive contributions that refugees and migrants make to the societies in which they live and also honor core European values: protecting lives, upholding human rights and promoting tolerance and diversity."
Swing, meanwhile, accused the West of having "refugee amnesia."
"We must remember that our organization and the U.N. High Commission for Refugees were created in 1951 precisely to take Europeans ravaged by the Second World War to safe shores in Canada, the U.S., Australia and elsewhere," he said.
"If we are creative in using our visa policies to give temporary protective status to everyone so there is a measure of support there ... then I think it is a manageable proposition," he said.
The number of people coming from North Africa across the Mediterranean into Italy dropped slightly this year, from 170,000 in 2014 to around 150,000.