The Commodity Futures Trading Commission voted Monday to make the rule final.
The agency had proposed the rule in October 2010 but held off adopting it after Jon Corzine, who led MF Global until last month, and others lobbied against it.
CFTC officials say the rule is aimed at reducing the risks and improve protection of customer funds. Among other things, firms will no longer be allowed to invest customers' money in debt of countries other than the U.S.
MF Global failed after making a disastrous bet on European debt. An estimated $1.2 billion or more may be missing from customer accounts.