Providing older people with quality long-term care (LTC) contributes to the Sustainable Development Goal (SDG) 3—Good Health and Well-being. Person-Centered Care (PCC) is the optimal approach that enhances the quality of life for older adults residing in LTC facilities. This study develops the Long-Term Care Unit Environment Assessment Tool (LTCU-EAT) to assess how LTC environments support PCC goals. The study was conducted in five steps. (1) Reviewing and revising assessment items based on existing literature; (2) Preliminary assessment and protocolling with expert opinions; (3) On-site assessments conducted by two raters among 21 LTC units across 13 facilities; (4) Reliability test of assessment items; (5) Scoring and reliability test of LTC unit samples. The LTCU-EAT, comprising 89 items distributed across 12 subscales within four themes, was developed based on a summary of 14 PCC goals. A total of 82 items (92.13%) demonstrated strong inter-rater reliability. The assessments of all LTC unit samples displayed good criterion-related validity. The LTCU-EAT is a valuable tool for conducting post-occupancy evaluation (POE) of LTC facilities, systematically evaluating the level of environment support for PCC, and providing empirical evidence for future research, policy and practice.