Duterte made the commitment at a meeting late Monday with Russian Prime Minister Dmitri Medvedev on the sidelines of a regional summit in Manila.
"I would just like to say that we have a working functional justice system here and they will get a fair trial and that they will be detained in a comfortable house," Duterte told Medvedev, according to an official transcript.
"Whether (the allegations are) true or not, we will find out but I will see to it that they are treated fairly and no additional stress on their person that will happen."
Yuri Kirdyushkin and Anastasia Novopashina were under investigation for drug smuggling but had not been convicted, Duterte said.
Duterte's 16-month rule has been dominated by a crackdown on illegal drugs in which police have reported killing 3,967 suspects.
Another 2,290 people have been murdered in drug-related crimes, while thousands of other deaths remain unsolved, according to government data.
Rights groups and domestic critics say the crackdown could be a crime against humanity, warning that police and hired assassins are carrying out mass murder.
Duterte has repeatedly railed against his critics and insisted human rights should take a backseat to eradicating drug traffickers.
He has also said he would be happy for all Filipino drug addicts to be "slaughtered".
Rights groups say the crackdown has also added pressure on the country's extremely over-crowded prisons, with inmates forced to endure abject living conditions.
In many cases prisoners have to take turns sleeping while waiting for years for their cases to go through the courts.
Duterte did not provide details on the two Russians' cases, and the Russian embassy and Philippine Drug Enforcement Agency spokesmen told AFP they could not immediately comment on the case.
Previous news reports have said Kirdyushkin was arrested with about 10 kilos (22 pounds) of cocaine at Manila airport on October 5 last year.
Manila airport customs police detained Novopashina in November last year after finding nearly 13 kilos of cocaine in her luggage, the reports said.
Russia's ties with the Philippines, a long-time US ally, have been improving dramatically under Duterte.
Duterte is a self-described socialist and fan of Russian President Vladimir Putin.
Duterte on Monday also thanked Russia for supplying guns and other weapons that he said helped defeat Islamist militants in the southern Philippine city of Marawi.
The official transcript of Monday's meeting did not include Medvedev's response on the two detained suspects.
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