The Electoral Commission has launched a public awareness campaign to remind people of the need to be registered.
Otherwise, they will not be able to vote on 7 May.
It has made a new television advertisement to remind voters they will be turned away from polling stations unless they are registered.
The commission will also use radio, press, outdoor and digital advertising to remind the public about the 20 April deadline.
Ann Watt, the head of the Electoral Commission's Northern Ireland office, said: "There has already been a great deal of interest and debate ahead of the UK general election, with voters keen to have their say.
"However, many people do not realise that they aren't registered and how easy it is to do so.
She added: "We don't want anyone to be turned away on polling day, so if you aren't registered take action now by downloading a registration form."
Voters can use the commission's website - www.aboutmyvote.co.uk - or can call the helpline on 0800 4320 712 to become registered.
The general public are being reminded that they must register to vote by 20 April if they want to take part in the general election.