A suspect in the murder of rapper G$ Lil Ronnie has been arrested ... TMZ has confirmed.

Forrest Hills Police Chief David Hernandez tells TMZ … Adonis Robinson -- a 24-year-old man -- was arrested in Livingston, Texas ... about 75 miles outside of Houston.

The Intelligence division of the FHPD was involved in the arrest ... though it's unclear how they tracked Robinson down.

We're told a second suspect -- a 21-year-old named Jakobe Russell -- is still at large.

As you know ... G$ Lil Ronnie and his five-year-old daughter R'Mani were shot to death at a car wash in Forrest Hills -- a suburb near Fort Worth, TX Monday. The two were spending time together to celebrate her birthday when cops say the two suspects jumped out of their white Kia and fired multiple bullets into the car, killing R'Mani and wounding Ronnie.

We obtained video from the scene which shows then men then chase Ronnie around the parking lot ... before killing him. Officers arrived at the scene, but they could not save the two.

The suspects were identified by law enforcement Wednesday ... and, Chief Hernandez called the two men cowards. A motive for the shooting hasn't been identified.