Rehn described himself as a "friend and admirer" of Turkey's best-selling novelist.
A year ago, Rehn visited Pamuk's home in Istanbul in a show of EU support for the writer, who was then facing charges of "insulting Turkishness". The case was dropped on a technicality in January.
Rehn pointed out that there are several other similar but lesser-known cases still pending in Turkish courts. Similar charges against another writer, Elif Shafak, were dropped late last month.
The EU has expressed concerns over freedom of speech in Turkey's membership talks.
Six of Pamuk's nine novels have been translated into Finnish, along with editions in some 40 other languages.