The Massachusetts senator, who is running for re-election in 2012, said Monday he applauded the move by Ryan, R-Wis., to initiate a necessary dialogue on the issue but he could not support the specific plan.
Under Ryan's proposal, for those under 55, Medicare would be transformed into a premium support plan over the long term, with people receiving federal dollars to purchase supplemental health insurance in a regulated private market.
"Our country is on an unsustainable fiscal path," Brown said in an op-ed piece published by Politico. "But I do not think it requires us to change Medicare as we know it. We can work inside of Medicare to make it more solvent."
"If Medicare is to survive for current beneficiaries and future generations, we must act," Brown said.
"The sooner Congress addresses this, the less painful it is likely to be -- but more difficult adjustments will be required if we delay."
Brown is joining another centrist, GOP Sen. Susan Collins of Maine, who announced last month she would not vote for the plan, The Hill reported. Sen. Olympia Snowe, R-Maine, also has been critical of the proposal but not said definitively which way she would vote.
Later Monday, Sen. Lisa Murkowski, R-Alaska, told Politico she has had to make a decision on the Ryan budget plan but "I'm not so sure about the components of his Medicare proposal."
Senate Majority Leader Harry Reid, D-Nev., is expected to hold a vote as soon as possibly this week on the plan.