4:05 PM PT -- Harry Styles is also paying tribute to Liam Payne ... saying he's "truly devastated" by Liam's death.

Harry says Liam's "greatest joy was making other people happy" ... and he's remembering Liam for living "wide open, with his heart on his sleeve." Harry also says Liam "had an energy for life that was infectious" and he was "warm, supportive and incredibly loving."

HS adds his heart breaks for Liam's family ... including his son, Bear.

2:08 PM PT -- Zayn Malik has issued his own personal tribute to late One Direction member Liam Payne ... expressing his sadness in a heartbreaking Instagram upload.

Alongside a photo of himself and Liam seemingly napping during their 1D touring days, Zayn shared his regret for never thanking LP for helping him through "some of the most difficult times," including when he was homesick at 17.

As he continued, Zayn remembered Liam as being "head strong" and "opinionated," admitting they "butted heads" at times. Nonetheless, Zayn said he "secretly respected" Liam for his give "no f**ks" attitude.

Zayn signed off his note by expressing his love for Liam, who he praised as "the most qualified" musician.

Louis Tomlinson is sharing his grief following the death of former One Direction bandmate Liam Payne ... posting a poignant tribute to his social media.

In the wake of Liam's death in Argentina Wednesday, Louis posted to Instagram, where he shared he is "beyond devastated" by his peer's untimely passing.

He wrote ... "Yesterday I lost a brother. Liam was somebody I looked up to everyday, such a positive, funny, and kind soul."

Louis recalled the early days of their friendship, when they were first partnered -- along with Harry Styles, Niall Horan, and Zayn Malik -- to form One Direction on UK's "The X Factor."

He noted ... "I was instantly amazed by his voice but more importantly as time went on I got a chance to see the kind brother I’d longed all my life for."

Louis went on to praise Liam's songwriting ability, hinting he and his late friend had discussed collaborating again in the near future.

He added ... "And for the record, Liam was in my opinion the most vital part of One Direction. His experience from a young age, his perfect pitch, his stage presence, his gift for writing. The list goes on. Thank you for shaping us Liam."

As Louis closed out his note, he had a message for the late singer ... confessing he is "struggling with the idea of saying goodbye."

He also vowed to be a presence in Liam's son Bear's life ... "I will be the Uncle he needs in his life and tell him stories of how amazing his dad was."

In addition to Louis' personal message, One Direction's official Instagram account released a statement on behalf of the surviving members of the band.

The note states ... "We’re completely devastated by the news of Liam’s passing. In time, and when everyone is able to, there will be more to say. But for now, we will take some time to grieve and process the loss of our brother, who we loved dearly."

The message continues, saying the bandmates will treasure their memories with Liam "forever."

The statement concludes ... "For now, our thoughts are with his family, his friends, and the fans who loved him alongside us. We will miss him terribly."

As TMZ previously reported ... Liam died Wednesday after falling from the balcony of his third-floor hotel room in Buenos Aires, Argentina. According to a 911 recording, Liam allegedly had a meltdown while intoxicated in his hotel room at the CasaSur Palermo Hotel before the fall ... and he was later pronounced dead at the scene.

Fans, family and celebrity friends have all paid tribute to Liam in the day since ... with many holding vigil outside his hotel.

He was only 31.

RIP