Nelson Cruz is officially a United States citizen ... and he's already itchin' to hit the voting booths!!
The Seattle Mariners superstar announced the big news by posing with his new American flag on Instagram Monday morning ... saying "Mission accomplished. Next stop: Registering to vote."
The 38-year-old grew up in the Dominican Republic, and has been playing in the MLB since 2005.
U.S. citizenship is becoming a trend with the Mariners -- Cruz's teammate Felix Hernandez (from Venezuela) just got his citizenship last month and was stoked about it!
"It means a lot,” Hernandez told the Seattle Times ... "Now, I am proud to be an American. Just happy, and so proud to be an American."
BONUS -- check out Cruz's stars and stripes kicks ... talk about swaggy AND patriotic.