Mr Blair earlier told the Cabinet he did not want ministers paying tribute to him, adding "that can be left for another day".
But as the meeting was breaking up, Mr Brown said he "did not think it would be right to let Cabinet finish without offering thanks to the prime minister".
He praised Mr Blair's "unique achievement over 10 years and the unique leadership he had given to the party, Britain and the world".
His comments were greeted by "much thumping of tables" by Mr Blair's colleagues, the prime minister's official spokesman told reporters.
Meanwhile, the BBC is running a comical poll on what Blair will be most remembered for:
- His international role
- Domestic policy
- Reforming Labour Party
- None of the above
"His international role"... what is it they are trying not to say? Rearrange these four letters to get the name of a country: RAQI.