"The X Factor" creator posted on Instagram, revealing he is "truly devastated" following the news about the singer -- who Simon tapped to join One Direction back in 2010.

Simon wrote to Liam directly in his statement, expressing ... "Heartbroken. And I feel empty. And I want you to know how much love and respect I have for you. Every tear I have shed is a memory of you."

Simon confessed it was "difficult to put in words" the entirety of his grief ... noting he was reflecting on their many memories together.

He went on to remember Liam as a "kind, funny, sweet, thoughtful, talented, humble, focused" person, who had an incredible talent for music.

Simon recalled his first meeting with Liam ... which occurred when the late One Direction alum was only 14 auditioning for "The X Factor" for the first time. While Liam didn't make it through, Simon credited Liam for coming back and auditioning again ... which led to the creation of 1D.

Simon said he met with Liam last year, when they caught up and talked about their years together ... with LP declaring "how proud" he was to be a dad.

He added ... "After you left, I was reminded that you were still the sweet, kind boy I met all of those years ago."

As he signed off his note, Simon said he'd be keeping Liam's family, friends and fans in his thoughts during this trying time.

Liam passed away in Buenos Aires, Argentina Wednesday after falling from the balcony of his third-floor hotel room. A police official later told The Associated Press the singer "had jumped from the balcony of his room." An investigation is ongoing.

Per local officials, Liam sustained multiple injuries from the fall, which caused him to die from "internal and external hemorrhage."

Though a toxicology report is still pending, an emergency call from staff has surfaced from moments before his death ... which claimed a guest at the hotel was "overwhelmed" by drugs and alcohol amid a meltdown.