The 26-year-old Cleveland superstar and Kara Maxine Kavajecz made their engagement public on Monday … sharing pics of the couple’s big moment in San Francisco.

The setup looked truly special … with Bieber setting up a romantic area overlooking the city and decked out with pillows, candles, wine and more.

“Forever my girl,” Bieber captioned his post with shots from iQphoto, which showed him on one knee and embracing his new fiancee.

“I always knew,” Kara added.

Bieber and Kavajecz first met years ago at the University of California, Santa Barbara … and have been inseparable ever since.

Of course, Biebs is one of the best in the sport — he’s 41-18 in his career with a 3.31 ERA.

The pitcher has been sidelined with an injury since mid-June with a shoulder strain … but it’s safe to say he’s been making great use of his time off the diamond.

Congrats to the couple!!!