Finnish President Alexander Stubb is up by 5 a.m. most mornings for triathlon training, a sauna—and, lately, a call from Donald Trump .

“Why are you up already?” Trump asked in a recent predawn conversation.

The two presidents have formed an unexpected bond, exchanging texts and calls at odd hours. The wonkish 57-year-old Finn might appear to have little in common with Trump: He champions the North Atlantic Treaty Organization, speaks five languages and warns about the threat from Russia.

Fortunately for trans-Atlantic relations, Stubb is also an excellent golfer.

His burgeoning friendship with Trump, which began on a West Palm Beach golf course almost five months ago, has vaulted the little-known Stubb into a back-channel role with the U.S. president—and made him a key conduit for European officials seeking to influence Trump’s summit with Russian President Vladimir Putin in Anchorage, Alaska, on Friday.

Working with officials in Berlin, London, Paris and Kyiv, Stubb has been picked to communicate Europe’s views to Trump, sometimes in back-to-back calls with Trump and Ukrainian President Volodymyr Zelensky .

“People know that we Finns don’t have a hidden agenda, and we’re also quite blunt,” Stubb told The Wall Street Journal in an interview on July 31. “I can communicate what Europeans or Zelensky think to Trump, and then I can communicate what Trump thinks to my European colleagues.”

During the Cold War, neutral Finland, which shares an 830-mile border with Russia, often served as a bridge between Washington and Moscow. It has since joined NATO and aligned itself more closely with the West.

Stubb has extensive experience negotiating with Russia and a familiarity with the U.S. that he picked up attending Furman University in South Carolina on a golf scholarship in the 1980s. He is, as he often tells Americans in a heavy drawl, “Finnish by birth, Southern by the grace of God.”

Trump has escalated criticism of Putin in recent months and aligned himself more closely with Europe’s call for bolstering Kyiv’s defenses and rejecting Moscow’s demands. In a notable shift, Trump promised last month to sell Europe billions of dollars worth of arms so that Ukraine could fend off Russian air and ground attacks.

Stubb doesn’t claim responsibility for the shift, and it remains to be seen if Trump will persist in pressuring the Kremlin toward a cease-fire. But senior European officials say Trump’s willingness to coordinate with European counterparts is a welcome reversal from just a few months ago.

Stubb has “been a conduit, a bridge, between Europe and Trump,” said Sen. Lindsey Graham (R., S.C.), adding that he often speaks to Stubb twice a day. “I would say that Alex has probably been the most out front day to day—texting with him, getting insight about what’s going on, giving advice.”

Stubb grew up in the Helsinki suburb of Lehtisaari, where his father worked as the chief European scout for the National Hockey League. At age 13, he visited New York City with his ice-hockey team, beginning a lifelong fascination with the U.S. and its politics.

He spent several summers in the U.S. and Canada, including an exchange year in Daytona Beach, Fla., so he could play golf year-round. Intending to become a professional golfer, he enrolled in Furman, in Greenville, S.C., immersing himself in U.S. politics and international relations, along with the Federalist Papers and Southern fiction.

After working in Brussels for the European Commission, Stubb entered politics in 2004 as a member of the European Parliament. A center-right conservative who argued for Finland to join NATO, he quickly rose through the ranks, returning to Helsinki to serve as foreign minister and later prime minister.

For Stubb, as for many Finns, the threat from Russia has a deeply personal dimension. Both his grandfather and his father were born in cities that Finland was forced to cede to the Soviet Union in 1940 and that are today part of Russia. Yet, for years, the country’s security strategy rested on straddling East and West.

Stubb quit politics in 2017 to go into finance and academia, partly because of his frustration with the firm public opposition in Finland to joining NATO, he said.

Seven years later, Russia’s invasion of Ukraine jolted Finland into joining the alliance and pulled Stubb back into politics. In his run for president, a post that makes him commander in chief, Stubb advocated for more security spending and closer ties to the U.S.

His confident style stood out in the otherwise low-key world of Finnish politics. His opponents accused him of arrogance, calling him out for wearing shorts and swearing during diplomatic meetings. He won the presidency last year in one of the closest elections in the country’s history.

A few months into his term, Stubb met Graham when the U.S. lawmaker was in Helsinki. They bonded over their South Carolina ties, golf and efforts to align U.S.-Europe policy on Russia.

When the two met for a drink at a security conference in Munich in February, Graham suggested they call Trump. “Alex used to play golf at Furman,” Graham told Trump, who suggested he and Stubb should play sometime.

Stubb, who hadn’t played golf in years, began to hit some practice balls at an indoor range to shake off the rust. A few weeks later, he booked a last-minute flight to Miami, joining Trump for breakfast at his West Palm Beach golf club, along with Graham, former South Carolina Republican Congressman Trey Gowdy and South African golf legend Gary Player.

Stubb’s golf game impressed Trump, according to others present. Between shots, the two leaders chatted about everything from Stubb’s student days in the U.S. to icebreakers, forestry and Finland’s World War II experience with Russia.

Over lunch, Trump asked if he could trust Putin. “You cannot,” Stubb said he replied, explaining that the Russian leader only understands strength, and advocating for tougher sanctions and a firm deadline for a cease-fire.

Trump gave Stubb a new set of clubs to replace the 20-year-old set he brought. A photo of the two posted on Truth Social showed them grinning and holding up matching trophies they had claimed in a member-guest tournament.

“He is a very good player,” Trump wrote. Stubb added diplomatically that Trump’s golf game is “much better than people think.”

So was Stubb’s lobbying. Hours after their victory, Trump for the first time harshly criticized Putin for prolonging the Ukraine war, saying in a TV interview that he was “pissed off” and “very angry” at the Russian leader’s rejection of a cease-fire.

Leaving Mar-a-Lago, Stubb phoned other European leaders on the way to the airport, flying to London to meet British Prime Minister Keir Starmer . He later briefed Zelensky, sharing that Trump’s patience with Putin appeared to be running out, Stubb said.

Trump and Stubb began to text and call each other regularly outside of formal meetings. Sometimes they text about golf tournaments, other times Trump has reached out before speaking to Putin. “Do you understand what I’m trying to do here?” he asked Stubb in April, after his sweeping new tariffs rattled global markets.

“If it is America First, make sure it is Finland Second,” Stubb says he told Trump.

Stubb said he has found Trump to be different in private from his public image, calling him a “listener” who can engage in “very frank conversations.”

Their contacts have become so frequent that Finnish diplomats in Washington joke that instead of reporting developments in Washington to Helsinki, they were hearing about it from the president.

European officials have noted that Trump responds better to hearing Europe’s views from the head of a small country. He admires Finland’s military and respects Stubb’s decades of experience dealing with Moscow, officials say.

“Because of our proximity, we have no illusions about Russia,” says Finnish Foreign Minister Elina Valtonen. “If Trump is a business guy, Putin isn’t. Whatever you give him in trade and investment, he’s happy to take it, but he will not back down an inch.”

Since meeting Stubb, Trump has also started praising Finnish ice-breaking ships, calling them the “king of icebreakers.”

In recent weeks, even U.S. officials have turned to Stubb. As U.S. lawmakers worked with experts to toughen Russia sanctions legislation last month, they appealed to Stubb to relay the changes to Trump, who had voiced reservations about the bill.

Ukrainian officials say Stubb has been an invaluable conduit for Zelensky, especially in the aftermath of his Oval Office clash with Trump. In July, Zelensky publicly thanked Stubb for “helping build a connection” with the American president on his behalf at a critical moment.

It is a role that Stubb couldn’t have predicted when he arrived in the U.S. with dreams of becoming a professional golfer. In those days, he said, his father told him that golf skills would come in handy, even if he never turned pro.

“I thought that was his way of telling me I’d never be good enough,” he said. “When I played with Trump, he sent me a message: ‘Told you so.’”

Write to Vera Bergengruen at vera.bergengruen@wsj.com