The organisation's Glasgow-raised director general, Ken McCallum, says the move will allow MI5 to “keep the country safe” by expanding its reach.
Users will be able to follow the Secret Service via @mi5official.
It plans to use to platform to bust popular myths and reveal never-before-seen material from its archives.
Mr McCallum said the service would still need to keep many of its secrets so it can operate undetected in dangerous environments.
“But the other half of the dilemma is that MI5’s ability to keep the country safe and resilient also depends on our reaching out to others who can help us, and whom we in turn can help,” he wrote in The Daily Telegraph.
“We owe it to the public to be constantly striving to learn and improve; and in our fast-moving world, with technology advancing at incredible speed, it would be dangerous vanity to imagine MI5 can build all the capabilities it needs inside its own bubble.”
The platform will be used for online Q&As with serving intelligence officers, as well as promoting career opportunities.
Mr McCallum expressed hope the service being on Instagram would allow it to recruit Britain’s best and brightest, regardless of background.
“We must get past whatever Martini-drinking stereotypes may be lingering by conveying a bit more of what today’s MI5 is actually like, so that people don’t rule themselves out based on perceived barriers such as socio-economic background, ethnicity, sexuality, gender, disability, or which part of the country they happen to have been born in,” he wrote.
It comes after Mr McCallum said in his first media engagement in October that he wants MI5 to “open up and reach out in new ways”, particularly in communicating with younger audiences.
“Much of what we do needs to remain invisible, but what we are doesn’t have to be,” he said.
“In fact, opening up is key to our future success.”
The agency follows GCHQ, which joined the Facebook-owned social network in October 2018.