The May 2010 Israeli assault on the Turkish ship the Mavi Marmara in international waters en route to Gaza sparked widespread condemnation and provoked a major diplomatic crisis between Turkey and Israel.
The Humanitarian Relief Foundation (IHH), which organized the first flotilla, said in a statement that activists from 12 countries had met in Istanbul and taken the decision to send the ships "in the shadow of the latest Israeli aggression on Gaza".
"As most governments are complicit, the responsibility falls on civil society to challenge the Israeli blockade on Gaza," it said.
The IHH would hold a press conference on Tuesday, it added. The group is considered to be close to the Turkish government.
Nine Turks died in the raid and one more died in hospital this year after four years in a coma.
As part of an ongoing criminal trial brought by the IHH and the victims' families in 2012, a Turkish court in May ordered the arrest of four former Israeli military chiefs involved in the raid.
Talks on compensation began a year ago after Israel extended a formal apology to Turkey in a breakthrough brokered by US President Barack Obama.
But they stalled after Prime Minister Recep Tayyip Erdogan, who sees himself as a champion of the Palestinian cause and is now the president-elect, stepped up his rhetoric against Israel in the wake of its assault on Gaza.
NJF/NJF