The allegations against the Nigerian national date back to February and March 2017, but the woman later withdrew her complaint to the Monusco peace mission.
The UN investigators “found that the allegation of sexual exploitation was substantiated and that the victim had been paid off by the alleged subject to withdraw her complaint,” said the UN spokesperson Farhan Haq.
The policeman, who had since moved on to serve in the UN mission in South Sudan, was recently sent home and UN officials have asked Nigeria to report on any disciplinary or legal action taken against him.
The UN peacekeeping missions are facing a damaging wave of allegations of sex abuse and sexual exploitation of civilians that they are mandated to protect in conflict zones worldwide.
Under UN rules, it is up to the troop- or police-contributing country to take action against their nationals in cases of misconduct in peacekeeping missions.
That has led to complaints that peacekeepers have not been held accountable in their countries for sexual abuse and exploitation of civilians while serving under the UN flag.
The UN Secretary General Antonio Guterres has vowed to toughen the response to allegations of misconduct by the peacekeepers.
Last month, the Security Council adopted a US-drafted resolution aimed at reinforcing measures such as repatriating peacekeepers and withholding UN payments to soldiers involved in misconduct.
The United Nations has 96 000 peacekeepers serving in 14 missions worldwide.