The cyber-intelligence listening post became the first of the country's spy agencies to join the online social networking service.
The Government Communications Headquarters has previously kept itself to itself but is moving towards greater transparency following revelations by fugitive US intelligence contractor Edward Snowden, who claimed GCHQ was conducting bulk data collection.
"It's a big step for the organisation as we become more open about the work we do to keep Britain safe," said an unnamed GCHQ spokesman.
"We want GCHQ to be more accessible and to help the public understand more about our work.
"We also want to reach out to the technical community and add our voice to social media conversations about technology", along with maths and cyber-security.
The Twitter account will be used to highlight events, publications, blogs and opinion pieces.
"Hello, world" was chosen as the first tweet as it is often the first thing computer whizzes learn to write in coding programmes.
GCHQ is based in a giant, ring-shaped building nicknamed "the doughnut" in the spa town of Cheltenham in southwest England.
Among the few dozen Twitter accounts being openly followed by GCHQ was the official James Bond one, along with government, law enforcement and royal accounts.
The US Central Intelligence Agency wasted no time in welcoming GCHQ to Twitter.
The agency's debut on the platform sparked some humorous tweets.
One Twitter user wrote: "'GCHQ is now following you' is going to really freak some people out."
Another said: "GCHQ joins Twitter as if they weren't already here."