Justin Bieber popped the question to Hailey Baldwin a nanosecond after getting back together in June, but we've learned they're not rushing to the altar ... not by a long shot.

Sources with direct knowledge tell TMZ ... Justin and Hailey are waiting until sometime next year to tie the knot. We're told it's not that they're unsure ... not unlike Miley and Liam, we're told they feel they've made the commitment, and that's the most important thing.

We're also told it's a mistake to view their engagement as jumping the gun. As one solid source told us, "Justin's been in love with her for a long time, so this didn't just come from out of the blue." We're told there were a number of times -- times the public doesn't know about -- when Justin and Haley got back together.

The upshot -- everyone thought Selena Gomez was the woman he always wanted, but even when he was with Selena ... Hailey was on his brain.