Trump read parts of the letter on Monday night during a campaign appearance in New Hampshire.
Trump read parts of the letter on Monday night during a campaign appearance in New Hampshire in which he told supporters that he had the support of both Belichick and Patriots quarterback Tom Brady.
Both are popular in New Hampshire, where the Patriots are the nearest NFL team and a regional darling after four Super Bowl crowns with Brady as quarterback and Belichick as coach, most recently to conclude the 2014 season.
"I've received a number of inquiries relative to a note I wrote to Donald on Monday," Belichick said.
"Our friendship goes back many years. And I think anybody that spends more than five minutes with me knows I'm not a political person. My comments are not politically motivated. It's friendship and loyalty to Donald.
"A couple weeks ago we had Secretary of State (John) Kerry in the locker room, another friend of mine. I can't imagine two people with more different political views than those two. But to me friendship and loyalty is just about that, it's not about political or religious views."
Trump told his rally in New Hampshire a portion of what Belichick wrote.
"Congratulations on a tremendous campaign," Trump said Monday in reading from the letter.
"You have dealt with an unbelievable slanted and negative media and have come out beautifully -- beautifully. You've proved to be the ultimate competitor and fighter.
"Your leadership is amazing. I have always had tremendous respect for you, but the toughness and perseverance you have displayed over the past year is remarkable. Hopefully tomorrow's election results will give the opportunity to make America great again. Best wishes for great results."
Hours after Trump was elected, Belichick explained his take on why he wrote the letter.
"I write hundreds of letters and notes every month," said Belichick.
"It doesn't mean I agree with every single thing a person thinks about politics, religion and other subjects. I have multiple friendships that are important to me. That's what that was about. It's not about politics."
Belichick then refused to answer questions about politics, repeatedly uttering only the word "Seattle," a nod to New England's Sunday game against the Seahawks.
Brady, who served a four-game ban to start the season over the "Deflategate" ball inflation level scandal, told reporters that "I'm just going to talk about football."
Brady, a long-time Trump pal who said previously it would be "great" if Trump were elected, brought his wife, supermodel Gisele Bundchen, into the matter.
"I talked to my wife. She said I can't talk about politics anymore," Brady said. "I think that's a good decision."