The primary budget surplus, which represents the public sector's excess revenue over expenditures before debt payments, came in at 6.188 billion reais ($2.66 billion) last month , central bank data showed on Friday.
That was the smallest amount for October since the central bank began tracking the primary budget balance in 2001, and was much less than the median forecast of 14 economists for a surplus of 9.75 billion reais, according to a Reuters poll.
The rapid erosion of Brazil's finances this year has alarmed financial markets, fanning fears of a sovereign rating downgrade next year that could scare off investors and further undermine a timid recovery in Latin America's largest economy.
President Dilma Rousseff is betting on extraordinary cash stemming from corporate tax settlements and an oilfield auction bonus to beef up the government's finances, which are under close scrutiny from investors.
Critics say the use of one-off income items undermines the government's credibility, already damaged by last-minute accounting gimmicks last year that improved fiscal results.
So far this year the government has accumulated a primary surplus of 51.153 billion reais, less than half its annual target of 111 billion reais.
The government will need to save some 60 billion reais in the last two months of the year to meet the target.
The country had a primary deficit of 9.048 billion reais in September, its biggest in nearly five years.
In the first ten months of the year, the country's public sector overall deficit rose to 143.769 billion reais, or the equivalent of 3.66 percent of gross domestic product.
The deficit was equivalent to 2.47 percent of GDP in the same period last year.