Boston topped the Baltimore Orioles 6-3 at Fenway Park to notch their 15th straight Major League Baseball win.
That tied the club record set in 1946 by a team led by Hall of Famer Ted Williams.
The streak then ended with a 5-1 loss to the Orioles in the nightcap.
"I think everybody knew it was going to end at some point," Red Sox interim manager Chad Tracy said.
"Now we've got to refocus."
The Red Sox got to work quickly in the opening game, postponed from Tuesday by rain.
They took a 4-0 lead in the first inning on a two-run single by Caleb Durbin and a two-run triple by Jarren Duran.
Duran delivered a sacrifice fly for a run in the third and Ceddanne Rafaela belted his ninth home run of the season in the fourth.
Tracy said his players had "accomplished something great" in matching the 1946 team and achieving the longest MLB winning streak since the St. Louis Cardinals won 17 straight in 2021.
Baltimore jumped on Red Sox starting pitcher Eduardo Rivera early in the second game, taking a 4-0 lead in the first inning and rolling home.
It was the Red Sox's first defeat since a 10-2 home loss to Washington on July 1.
The streak marked a stunning turnaround for Boston, who were floundering at 32-46 on June 24 but are now approaching the trade deadline with a playoff berth in their sights.
"Not only did we have a run of 15 wins, we found out who we are - and we're a good baseball team," Tracy said.