Kanye & Kim K. Yeezus! Engaged with a 15 Carat Rock

Kim Kardashian is officially on lockdown ... again -- Kanye West pulled off a huge birthday surprise by renting out an entire baseball stadium and asking Kim to marry him Monday evening.

Naturally, Kanye went super-baller for the proposal -- sources connected to the couple tell us Yeezy flew Kim to San Francisco, blindfolded her, and whisked her off to AT&T Park ... home of the Giants.

Ye secured the ballpark for the event -- and we're told he had a full orchestra there playing Lana Del Rey's "Young and Beautiful" ... one of Kim's favorite songs.

We're told Kanye went traditional for one part ... getting on one knee in the middle of the field and popping the question with a 15 CARAT DIAMOND RING!!! (15ct. was not the traditional part).

Read more: http://www.tmz.com/2013/10/21/kanye-west-kim-kardashian-engaged-to-be-married-san-francisco/#ixzz2iSxCVdC4
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Shocker ... she said yes. Family and friends then joined the couple on the field to celebrate.

The engagement comes 4 months after Kim and Kanye's daughter North was born -- and 18 months after they went public as a couple.

Monday was Kim's 33rd birthday. This will be her third shot at "til death do us part," and Kanye's first.

BTW -- In case you're interested, it costs $35k to rent out the field.  We're guessing he can afford it.