This is a Phase 2b, open-label, non-randomized, single arm study to evaluate the safety, and efficacy of HBI-8000 40 mg BIW in patients with relapsed or refractory ATL (R/R ATL). HBI 8000 will be administered orally approximately 30 minutes after any regular meal twice a week. There will be 3 to 4 days between dosing. A treatment cycle is defined as 28 consecutive days. HBI-8000 administration will be continued until disease progression or unacceptable toxicities are observed despite appropriate dose reduction or treatment interruption.