Rowsey hit six treys and scored 31 points in a 91-87 loss to No. 6 Xavier in the conference opener.
Georgetown, which started the season 10-1 under first-year coach Patrick Ewing, lost to Butler in double overtime in the conference opener and now is 0-2 in conference.
The Golden Eagles got an 8-0 run in the first half, with two Jamal Cain 3-pointers bookending a Sacar Anim dunk, to take a 25-14 lead with under six minutes left, but the Hoyas battled back. Jessie Govan hit three-straight free throws with a second left to make it 34-27 at intermission and cut the lead to three, 34-31 barely a minute into the second half.
Rowsey hit three-straight 3-pointers to put Marquette up 49-36 and the Golden Eagles cruised home.
Markus Howard scored 13 points for Marquette (10-4, 1-1).
Marcus Derrickson scored 20 points and grabbed eight boards to lead Georgetown, with Kaleb Johnson adding 12 points and Govan contributing 11 points and 12 boards.