Mr Brown was the MP with the highest earnings from outside Parliament in 2012-13, but a spokesman said that all of the money went directly to charity or to fund charitable work by the ex-PM and his wife Sarah.
He has also chosen not to accept the traditionally generous pension given to former prime ministers, said the spokesman.
According to analysis by The Guardian, MPs declared earnings of more than £7m in the Commons Register of Interests from outside jobs and directorships in the last session of parliament.
Some 295 of the 650 MPs declared some form of income, and 20 made more money outside parliament than their MP's salary of £65,738. The total declared by Conservative MPs came to £4.3m and by Labour MPs £2.4m - more than half of it accounted for by Mr Brown, who listed a string of speaking engagements around the world.
A spokesman for Mr Brown said: "Mr Brown personally does not receive a penny from speeches or writings. He has thus no 'second income' and does not benefit from 'outside earnings'. All the fees from speeches and writings are not paid to him and all the money goes directly to charity or to fund charitable work by himself and his wife.
"Mr Brown's only income is his salary as an MP, because he has also renounced the pension conventionally paid to all former prime ministers."
Under parliamentary rules, MPs are allowed to take on outside work, including company directorships and consultancies, so long as all income is declared in the Commons register and their work does not involve lobbying parliament.
Other top earners included Conservative MP for Sleaford and North Hykeham Stephen Phillips, who declared £740,000 from work as a barrister. Mr Phillips said his outside work helped him keep in touch with "the real world", adding: "What matters is whether or not I do my job as an MP and how well I do it."
Tory MP for Torridge and West Devon Geoffrey Cox declared £417,000 from work as a barrister, Conservative MP for Mid-Sussex Nicholas Soames earned £305,000 from directorships and consultancies, former defence secretary Sir Malcolm Rifkind earned £276,000, Labour's former chancellor Alistair Darling £263,000, Tory MP for Wokingham John Redwood £238,000 and Labour's ex-home secretary Jack Straw £183,000.