The United Nations' refugee agency (UNHCR) says more than one million migrants braved the seas in 2015 to reach Europe, nearly five times more than in the previous year.
About half of the migrants who made the perilous journey came from Syria, while Afghans accounted for roughly a fifth, the United Nations High Commissioner for Refugees agency said in a statement on December 30.
Most of them made their way to Europe on the Aegean Sea to Greece from Turkey, the statement said.
Nearly 4,000 people were believed to have drowned before reaching shore, the UNHCR said.
The influx of migrants has caused tensions across Europe, prompting some nations to introduce tougher border controls.