The Sankei Shimbun reported Sunday the signed letter from Trump encouraged the family of Keiko Arimoto, who was abducted at the age of 23 in 1983. Trump promised victory for Akihiro Arimoto, the father of the abductee.
"Akihiro -- I am working hard for you. So is [Prime Minister Shinzo] Abe. You will win -- great to see you!" the letter read.
Last week, Trump praised North Korea's Kim Jong Un and his "beautiful" letter.
The message from Trump was a response to a letter Arimoto sent to the president, expressing the Japanese national's hopes for the return of people kidnapped to North Korea.
A dozen abductees are still officially recognized by Tokyo.
Arimoto, 90, a member of the Association of Families of Victims Kidnapped by North Korea, has met with Trump twice, first in November 2017 and again in May of this year, when Trump visited Japan for a summit with Abe.
Arimoto sent Trump a letter through the U.S. embassy in Japan each time, according to the Sankei.
During their meeting in May, Trump reportedly said the issue of abductions is a "very sad story." The president also said he would send a letter through diplomatic channels to Arimoto.
Arimoto "shed tears" after receiving Trump's letter, according to the Sankei.
"This is the last chance to resolve the issue of kidnappings," Arimoto said.
Trump is scheduled to return to Japan for the G20 Summit in Osaka on June 28.
Ahead of the event, police in Osaka are trying to identify a suspect in what appears to be a murder and robbery of a local police officer, the Nikkei reported Sunday.
The police officer was stabbed multiple times on the left side of his chest and his handgun is missing, according to the report.
The suspect is still at large.