The union successfully appealed a previous decision not to grant the status which allows organisations and individuals to be legally represented at the inquiry and have questions asked on their behalf.
General secretary Michelle Stanistreet said in a statement: “During the previous Leveson sessions, newspaper editors and proprietors have dominated and they have tried to set their own parameters for the inquiry.
“However, such executives represent a tiny minority of the industry and have a clear vested interest – the NUJ represents those journalists who make up the vast majority of a vibrant and diverse industry, and it is vital their voices are also heard and given serious consideration.”
Michelle added that the union “can provide the inquiry with knowledge and expertise that spans the breath of the media industry”.
“We speak on behalf of ordinary journalists who spend their lives working for different media organisations, including broadsheet and tabloid newspapers, at national, regional and local level,” she said.
The Telegraph Media Group and Trinity Mirror have also been granted core participant status.
The inquiry was set up by Prime Minister David Cameron in the wake of the phone-hacking scandal to look into whether the industry needs a new regulatory framework.