The Portuguese superstar is said to have contacted Barca centre-back Gerard Pique and asked him to speak to Nou Camp boss Xavi about a move, APA reports citing the Sun.
His agent Jorge Mendes is also already negotiating terms with Barca president Joan Laporta, according to Mundo Deportivo.
Ronnie is aware the cash-strapped Catalans are looking for a goalscorer.
He is said to have contacted Xavi 'a few days ago' thanks to an introduction by his former United team-mate Pique.
Mendes has a good relationship with Laporta and the pair are already discussing the viability of the move.
Several high-profile Barca execs were said to have thought the transfer of the Real Madrid legend was a joke when they first heard.
Ronaldo is said to be unhappy at Old Trafford with the team still struggling despite his added goals.