Don Toliver was arrested on suspicion of DUI this week -- but it sounds like he was an easy customer for cops ... 'cause they didn't even take him to jail.

According to a report from California Highway Patrol -- obtained by TMZ -- the rapper was pulled over for speeding early Tuesday morning near the San Fernando Valley here in SoCal -- and cops say they had reason to believe he might've been impaired behind the wheel.

The report says that officers observed signs of impairment but instead of throwing him in handcuffs ... Toliver was simply cited and released in the field.

While there's no explicit mention of another person with him -- law enforcement sources tell TMZ Don was released to the custody of another party who was sober and with him.

Unclear who exactly that was -- but the officers note DT was cooperative throughout the entire incident.

Still, not great news for Don, especially since he just welcomed a baby with Kali Uchis about a month ago. Definitely a headache he'll have to deal with -- but luckily, nobody was hurt.