It didn't let up until three hours later, not long after Wednesday night's game had been postponed following a delay that began immediately after Boston took a 5-0 lead in the second inning.
All statistics will be wiped out, including three home runs the Red Sox hit off Dylan Bundy. It was the 44th postponement in the majors this year.
The game will be made up as the opener of a day-night doubleheader on Aug. 11 at 1:05 p.m.
''We have some off days in there, so we're good,'' Boston manager Alex Cora said before the makeup date was announced.
Almost immediately after Mookie Betts ripped a three-run shot to make it 5-0, an intense shower caused the umpires to call for the field to be covered by a tarp.
Following a delay of 2 hours, 33 minutes, the game was called.
Along with Betts, Boston's Andrew Benintendi and J.D. Martinez had home runs washed away by the rain. Benintendi and Martinez connected on the fourth and eighth pitches thrown by Bundy.
The postponement also ruined a promising start by David Price, whose night on the mound consisted of a 1-2-3 first inning.
The major league-leading Red Sox came into the game with a 10-2 record against the last-place Orioles, who were playing their first game without closer Zach Britton. Baltimore shipped Britton to the Yankees late Tuesday night in a trade for three minor leaguers.
The rainout ruined Boston's chance to take a six-game lead over second-place New York in the AL East. Earlier in the day, the Yankees lost to Tampa Bay 3-2.
The day wasn't a total loss for the Red Sox. Hours earlier, Boston obtained right-hander Nathan Eovaldi in a trade with the Rays. Eovaldi is expected to join a rotation that is currently without injured left-hander Eduardo Rodriguez, who's still in a walking boot after spraining his right ankle on July 14.
After being designated for assignment, RHP Chris Tillman (1-5, 10.46 ERA) has opted to become a free agent rather than toil in Baltimore's minor league system. ''He's done a lot of great things here, just like the two guys we lost,'' said Orioles manager Buck Showalter, referring to the departure of Britton and Manny Machado Britton via trades. The trio helped Baltimore reach the postseason just two years ago.
Former Orioles star Eddie Murray has accepted a post as special adviser to the team, the same title given earlier this month to fellow Hall of Fame member Brooks Robinson. Each will serve as a liaison to the community.
Red Sox: LHP Brian Johnson (1-3, 3.81 ERA) makes his sixth start and 27th appearance of the season Thursday night, facing Minnesota in the opener of a 10-day homestand that concludes with four games against the Yankees.
Orioles: Alex Cobb (2-13, 6.17 ERA) takes on his former team Thursday night when Baltimore begins a four-game series with Tampa Bay. Cobb is 0-2 in two career starts against the Rays.
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