Arenado, 28, took the first pitch he saw from Baltimore's Andrew Cashner in the bottom of the third inning over the left field wall Saturday. The three-run home run, Arenado's 14th of the year, gave Colorado a 4-3 lead.
"Two hundred homers is really cool," Arenado said, according to MLB.com. "I'm really happy about it. I'm going to enjoy it."
Arenado is on pace for another All-Star season. The four-time All-Star hitting .333 and has 42 RBIs in 49 games this year.
Arenado's milestone home run came a day after teammate Trevor Story became the fastest shortstop in history to reach 100 career home runs. Story joined the club in only 448 games.
"He's such a special guy, a special talent," Story said of Arenado. "No one works harder. It doesn't surprise me. I've seen it for a while now. It's crazy to say, this has become normal -- him doing special things like this."
The Rockies fell to 23-27 with Saturday's loss. Colorado trails the Los Angeles Dodgers by 10 games for first place in the NL West Division.