Brazilian-born Deco, who played for Portugal in two World Cups, was released by English champions Chelsea at the weekend and is expected to make his 'Flu' debut in the Rio derby against Vasco da Gama on August 22.
"I owed myself this, it was a big dream to be able to return to Brazilian football after 13 years," said the 32-year-old who had a short spell with Corinthians before joining Benfica in 1997.
"There were big difficulties getting released from my contract with Chelsea but I thank Fluminense for keeping the door open and making the effort to bring me back," he told reporters.
"I will certainly pay that back with a lot of effort on the pitch to win titles," added Deco who was born near Sao Paulo.
"I was lucky to join the leaders and I'm happy to be at a very good club... with a fantastic coach (Muricy Ramalho) who is a big name in Brazilian football."
Ramalho was first choice to replace Dunga as Brazil coach after last month's World Cup but Fluminense refused to release him.
"I want to prepare properly. I've been doing fitness work for a week under the orders of Fluminense's physical trainer so I can make my debut as soon as possible," added Deco who will wear his favourite number 20 shirt.
Deco, full name Anderson Luis de Souza, won the European Champions League with Porto in 2004 and again with Barcelona two years later.
He spent two seasons at Chelsea, having been signed by Brazilian Luiz Felipe Scolari, but injuries and poor form blighted his time in the Premier League.
Deco quit international football after last month's World Cup.
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