This is a 2-part, randomized, placebo-controlled, double-blind, Phase 2 study to evaluate the safety, tolerability, efficacy, and pharmacokinetics (PK) of RTA 901 in qualified participants with Diabetic Peripheral Neuropathic Pain (DPNP). Each study part will be randomized into 3 treatment arms; 2 different doses of RTA 901 and RTA 901-maching placebo. The doses of RTA 901 in Part 2 will be selected based on the Exposure-Response (E-R) analyses of data from Part 1. The duration of each part of the study will be approximately 20 weeks, including a Screening period of up to 2 weeks, a Run-in-period of 2 weeks, a Treatment period of 12 weeks, and a Follow-up period of 4 weeks. All participants in Part 1 and Part 2 of the study will follow the same visit and assessment schedule. Eligibility will be assessed during the Screening and Run-in-periods.