Mr Morrison announced an inquiry on Sunday into what he described as an "alarming and disturbing" spike in elder abuse and poor standards in the aged-care sector.
The prime minister's decision jars with comments from Aged Care Minister Ken Wyatt, who has said a royal commission would be a waste of time and money.
Critics believe the inquiry is a cynical ploy months out from an election, particularly given the coalition has been in power for five years.
"I've been the prime minister three weeks, I think that's a pretty quick time," Mr Morrison told the Nine Network on Monday.
"I can take responsibility for my actions as prime minister, and that's what I'm doing."
He says the royal commission is needed to understand the full extent of problems in aged care.
"It's not just in the profit or the not-for-profit sector, it's not just in rural centres or urban centres or large centres or small centres," he said.
"Our work shows that there is a problem potentially and actually in each of these areas, so we want to get to the bottom of it."
"I don't want to fight about it, I just want to fix it."
Labor frontbencher Jim Chalmers believes the prime minister is crying crocodile tears, having cut millions of dollars from the aged-care sector while he was treasurer.
"Older Australians deserve much better than a prime minister that cuts funding to aged care and then lies to them about it," Mr Chalmers told reporters in Canberra.
"If Scott Morrison is going to finally take responsibility for the mess that is aged care, he should start by 'fessing up to the impact his cuts have had on quality in the system.
"Scott Morrison is the architect to the cuts to the aged-care system."
The inquiry comes after audits at some facilities revealed a dramatic increase in non-compliance and abuses in the sector.
There was a 177 per cent increase in the number of aged-care homes where a serious risk to residents was identified in the past financial year.
There was also a 292 per cent increase in the number of facilities that refused to comply with rules.
The royal commission will look at the quality of both residential and home aged care, including how young Australians with disabilities are cared for in residential facilities.