"He came to say goodbye, given that he will retire next month," Renan Calheiros told reporters Thursday after a private meeting with the jurist.
"It was a surprise and we're very sorry, since he's one of the best models the country has," the senator added.
The chief justice met earlier Thursday with President Dilma Rousseff to inform her of his decision.
Barbosa, 59, was the first black jurist to head Brazil's Supreme Court, elected by his 10 fellow justices in October 2012.
He earned unprecedented fame among Brazilians for his inflexible character during the so-called "trial of the century" centring on a 2005 corruption scandal that rocked the administration of then president Luiz Inacio Lula da Silva.
Twenty five of the 37 defendants were found guilty, in large measure due to the extensive work done by Barbosa, who handed down very harsh sentences to those convicted, including Lula's one-time presidential chief of staff, Jose Dirceu.
Also convicted and sentenced to prison was Jose Genoino, a former chairman of Lula and Rousseff's centre-left Workers Party.
The trial garnered Barbosa enormous popularity in a country where it is often said that corruption is never punished and it made the majority of Brazilians see him as a champion in the struggle to clean up public management.
However, it also made him the target for Lula's continuous criticism, since the former president felt that the trial was "80 percent political and just 20 percent legal".
Several opposition parties toyed with the idea of proposing Barbosa for a Senate candidacy or even the presidency in the October elections, but he has always denied having politics as part of his near-term plans.
However, he said last year that he would not rule out running for office in 2018.