More than 50 PSNI officers have been disciplined after blunders in missing persons investigations in a six-year period.
Staff from the Police Ombudsman's Office found missed opportunities to gather evidence, a failure to properly assess reports and problems with supervision and searches.
Police Ombudsman Dr Michael Maguire said the PSNI needed to rethink how it handles missing persons cases.
It follows an investigation into how police responded to reports of a missing Chinese national.
Geng Feng Shi (23) was found on March 27, 2015, in the River Lagan at Belfast Harbour - three weeks after he was reported missing by relatives.
Ombudsman investigators found a series of lapses in the investigation.
Between 2010 and 2016, the watchdog has investigated six separate missing person reports and recommended a total of 55 officers be disciplined.
Dr Maguire said: "I'm aware that the PSNI get on average 13,000 reports of people missing each year, very few of which are referred to my office.
"Yet some of the failings we identified in 2010 were still happening in 2015, which represents a real risk to public confidence in the way police deal with missing persons investigations.
"Many of the officers we spoke to have talked of difficulties in balancing the resources needed in responding to missing person reports with those for other urgent enquiries.
"Police officers have to carry out proper risk assessments in each case if there are to be effective investigations.
"It is also important that police ensure these investigations have the necessary resources which are then properly allocated and tasked."
Dr Maguire said he has made a series of recommendations, including setting up a central resource to provide specialist assistance to officers across Northern Ireland.
In the Geng Feng Shi case, Ombudsman investigators found that although it was likely that he was no longer alive by the time he had been reported missing, the way in which police responded to concerns about him included common failures.
On March 3, 2015, Geng Feng had been assessed by a psychiatric doctor in hospital.
Police had seen him twice behaving abnormally - lying in the street singing and at a cemetery refusing to communicate.
But when his family rang on March 5 to say they could not contact him, he was not logged as a missing person.
Dr Maguire added: "This decision was a fundamental failure, given PSNI guidance. It resulted in an unacceptable delay in police undertaking proactive and robust enquiries."
On March 20, police contacted Geng Feng's family in China - 15 days after he had first been reported missing.
The family said they believe he was depressed and normally kept in regular online contact.
As a result of this conversation, police changed Geng Feng's status to that of a high-risk missing person.
CCTV footage showed Geng Feng near Lanyon Place, close to the Waterfront Hall and the River Lagan at around 7am on March 4. His body was later recovered from the river.
The investigation recommended 14 officers be disciplined for failings linked to the case.
PSNI Assistant Chief Constable Alan Todd said new guidelines have been issued.
"This case illustrates that missing person reports present policing and partner agencies with significant challenges that need to be managed in a manner that is consistent and commensurate with the risks," he said.
"This is a challenge which continues to grow due to the sheer scale of missing person reports and the complexity of many of the cases."
Mr Todd said that in the 12 months to April, the PSNI investigated 12,821 reports of missing persons, an increase of 5.2% from the previous year.
He said the figure is increasing year-on-year.