The Historical Institutional Abuse Inquiry is to investigate three additional state-run institutions.
There will also be a separate examination of the activities of paedophile priest Brendan Smyth.
The inquiry has been investigating 13 institutions to date.
However, Hydebank Young Offenders Centre in south Belfast and two former homes in Londonderry - Fort James and Harberton House - will now also be examined.
Both Derry establishments were controlled by the former Western Health and Social Services Board and have since closed.
The extended scope of the inquiry was announced on Monday by its chairman Sir Anthony Hart.
Smyth was at the centre of one of the first paedophile priest scandals to rock the Catholic Church in Ireland.
He was eventually convicted of dozens of offences against children over a 40-year period.
Smyth died of a heart attack in prison in August 1997, just a month into his 12 year prison sentence.
The HIA inquiry was set up in 2013 to investigate child abuse in residential institutions in Northern Ireland over a 73-year period, up to 1995.
In total, the inquiry is expected to hear from more than 300 witnesses during the course of the public evidence sessions.
It is required to complete its hearings and all investigative work by mid-summer 2016, and has to submit its report to the Northern Ireland Executive by January 17 2017.
Among its recommendations could be compensating victims who have alleged they were abused.
Meanwhile, the Sisters of Nazareth have apologised for abuse by some nuns at two former children's care homes in Belfast.
Sister Brenda Mc Caul, a representative from the Sisters of Nazareth headquarters in London, told the inquiry on Monday morning that the congregation accepts that there is "obviously overwhelming evidence that some sisters, some sisters, did use excessive punishment on children".
She added: "I think we have to turn to these people who we have hurt and humbly ask their forgiveness for our trespasses. "Humbly ask their forgiveness of our trespasses of the past."