The 27-year-old has agreed a contract until the end of the season with a view to a longer-term deal.
"I'm very, very happy to join Middlesbrough," said Ricardinho.
"I've spoken to Juninho and Doriva and they have said only good things about the town and the set-up here.
"I need five or six days' training to get my fitness to the right level but I hope I will be available for selection on Saturday.
"The manager here told me I will get my chance but I will have to work for it.
"I've been looking to play in England for two years now so this is a very exciting challenge for me.
"Although my contract is only to the end of the season, I hope to impress the manager enough to earn myself a contract beyond that."
The former Sao Paulo man trained with his new team-mates on Monday morning and then passed a medical - after he agreed terms.
Boro gained international clearance late on Monday and Ricardinho is now free to make his Premiership bow on Saturday against Blackburn.
The Brazilian holds EU citizenship and did not require a work permit, which enabled a quick deal to be done.
Skysports.com revealed the news of Ricardinho's arrival and now any fears that the move would not be completed, have been allayed.
"We are a little bit short in the central of the midfield area, we needed cover in that area," said Boro boss Steve McClaren on Sky Sport News.
"We have a suspension to George Boateng coming up and Doriva has had a few injuries, so that was the thinking behind it.
"He has come highly regarded, from both Juninho and Doriva.
"I have seen him and I like him a lot.
"We have got ambition and that showed by the type of player we went for."
