In a series of heartfelt posts, RTK shared his love, old pics and text messages between him and Takeoff ... revealing they used to share clothes and shoes back in the days before they could buy whatever they want.

Takeoff, along with Offset, actually partied with RTK during his NYC birthday bash back in July -- when a Migos split seemed to be going down -- proving Takeoff was the bridge of peace between Quavo and Offset.

As we reported, Takeoff was fatally shot early Tuesday morning when Quavo got into a heated argument with some other guys outside a Houston bowling alley.

Police say 2 other people were wounded in a hail of gunfire -- at least 10 shots -- and they're both hospitalized with non-life-threatening injuries.

RTK also referred to Takeoff as his "twin," and someone who, obviously, did not deserve this cruel twist of fate.

Well said ... Takeoff was only 28.