Kendall Jenner Posts Sappy Love Letter ... From Ben Simmons?!

Is Ben Simmons secretly the sweetest, sappiest boyfriend ever???

Maybe ... 'cause Kendall Jenner posted a SUPER romantic, handwritten love letter on social media Monday (but blacked out the name of the sender) ... and signs point to the 76ers point guard.

The letter begins, "Dear Kendall, We may be miles apart but my love for you grows stronger by the minute. My innermost feeling is of pure love for you."

"I will love you until death separates us. Right now this separation is killing me, but in my heart I know that nothing would come close to separating us."

It continues, "We are united into one forever, body and soul and I want you to know that I love you from the deepest part of my heart."

It's signed with, "Love," but Kendall blocked out the author ... which begs the question -- IS IT REALLY FROM BEN?!?!

Seems it could be ... the two have gotten super serious the last few weeks, ice skating, grabbing bites to eat and hanging out after Sixers games.

Plus ... Philly WAS just on a road trip in Toronto and then Detroit ...

So, is Simmons the author?? Here's a look at his handwriting -- you be the judge.