Though the Red Sox had no comment late Wednesday night, agent Scott Boras told ESPN.com's Jerry Crasnick that the contract terms were all agreed upon, and that Matsuzaka passed his physical at Massachusetts General Hospital.
"We finalized the deal when he arrived in Boston," Boras told ESPN.com. "We came out of it with what I felt was the right decision for the player."
That was a capper to a wild day that spanned 3,000 miles before the Red Sox finally closed the deal with Matsuzaka.
Boston's pursuit of Matsuzaka hit a peak on Wednesday, when Matsuzaka boarded a private plane owned by Red Sox owner John W. Henry and flew from Southern California to Bedford, Mass., landing at roughly 5:20 p.m. ET.
The sides continued to negotiate -- as assistant general manager Jed Hoyer put it -- from 35,000 feet above ground, and, according to Boras, closed the deal with Sox general manager Theo Epstein shortly after arriving.
The last significant hurdle was the physical, which was completed more than 24 hours before the midnight deadline at the end of Thursday, at which time Boston's negotiating rights to the pitcher would have ended.
Matsuzaka traveled coast-to-coast with Boras, Epstein and Sox president/CEO Larry Lucchino.
This, after Epstein and Lucchino had made a surprise -- and unsolicited -- visit to California on Monday to get a deal done with Boras and Matsuzaka. Tuesday was a pivotal day, as the sides had numerous negotiating sessions, and were believed to have stayed up for most of the night.
It was quite a scene at the Hanscom Air Force Base in Bedford, where Matsuzaka touched down in Massachusetts for the first time. There were roughly 50 Red Sox fans on hand to greet Matsuzaka as he disembarked. The pitcher went straight for the back seat of an SUV, which was escorted by a state police car.
With a wide smile, Matsuzaka gave several friendly waves to the fans who hope to be cheering his every pitch at Fenway in 2007 and beyond.
There was a large media gathering there for the landing of the plane, but there were no interviews before the entourage gathered into the car.
Upon arriving in California on Monday, Epstein promised that the Red Sox contingent would be headed back to Boston via Henry's plane on Wednesday, and the goal was that Matsuzaka and Boras would be on board.
Mission accomplished, but not without some anxious moments.
Earlier this week, Epstein, Lucchino and Henry all seemed frustrated at the pace of the negotiations and the fact that they had to make a second offer before Boras countered the first offer.
But deadline deals are nothing new for Boras, who has taken place in as many drama-filled negotiations as any agent in sports.
Matsuzaka, 26, is widely revered by baseball fans of Japan, and for good reason. He has a record of 108-60 with a 2.95 ERA over his eight-year career with the Lions, and last March, he was named the Most Valuable Player of the inaugural World Baseball Classic.
"In Japan, he's known as the national treasure," Boras said earlier this week. "Here, he will be known as Fort Knox."
The Red Sox, health permitting, could wind with one of the best rotations in baseball, anchored by Curt Schilling and Matsuzaka, and filled out by Josh Beckett, Jonathan Papelbon and Tim Wakefield.