The 27-year-old MLB player posted a picture of himself in a mask featuring a Blue Lives Matter flag ... just days after people accused a Minneapolis cop of killing Floyd by putting his knee to the man's neck.

The mask -- which had a black and grey American flag with a blue stripe in it -- caused tremendous online backlash for Kepler ... but the Berlin, Germany native brushed off the criticism.

"✌️wasn’t aware what the mask supported. still not into politics" Max wrote.

But, that only caused MORE criticism for the MLB player ... and he finally addressed the issue formally in a lengthy statement on his social media page Thursday.

"Earlier, I posted a photo of me in a mask that was sent to me by a company, to wear during the pandemic," Kepler said. "I had no idea that the mask had any underlying inferences -- I simply thought it looked good."

"After I was informed, I immediately took down the post. I take complete responsibility for not knowing what I was wearing. I am truly sorry for the hurt and pain my actions have caused, especially now."

Kepler continued, "Racism has no place in our world and I do not in any way support the actions that we all witnessed that led to George Floyd’s passing. My sympathies are with the Floyd family."

The Twins have not addressed Kepler's picture directly ... but did release a statement of their own Thursday, saying, "We are deeply saddened by the tragic and senseless death of George Floyd Monday night in Minneapolis."

"The Minnesota Twins send our deepest sympathies to the Floyd family, and join our Twin Cities community in mourning."