The new encrypted system will take effect in March 2014
The South American country decided to take the measure to protect official communications from US snooping, reported Xinhua.
Documents leaked by former US National Security Agency contractor Edward Snowden show US intelligence agencies regularly intercept and collect data on millions of citizens and companies in Brazil and other countries.
Brazilian President Dilma Rousseff and German Chancellor Angela Merkel were among those spied on.
The new encrypted system, announced last month and created by the government's Federal Data Processing System (Serpro), will take effect in March 2014. It does not apply to state-run companies such as oil giant Petrobras that have their own systems.
The decree states that government communications should be transmitted through "telecommunications networks and information technology services provided by organisations or bodies belonging to the public federal administration."
Government contracts with technology service suppliers will be exempt from the usual bidding process, in order to protect national security, it says.