Frontex cited worsening weather and more restrictive border-control policies along the Western Balkan route as the apparent reasons for the drop.
Some 108,000 people arrived in Greece in November, down from 150,000 in October.
The Warsaw-based agency said 715,000 migrants arrived on the Greek islands through the end of November, 16 times more than in the same period last year.
Most of the migrants are Syrians, followed by Afghans and Iraqis.
Frontex said the number of migrants crossing into Italy from Libya decreased for the fourth straight month.
EU agency reports drop in migrants to Greece
15 December 2015
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The European Union`s border control agency says the number of migrants arriving in Greece fell sharply in November, the first month-on-month drop in a year that has seen record numbers of asylum-seekers arrive in Europe.
