The Sitting Balance Scale (SBS) measures sitting balance for frail older adults who are primarily nonambulatory. Purposes were to generate items representing different functional abilities of sitting balance, reduce the number of items to make a clinically useful tool, refine the assessment, and establish reliability and validity. The study was conducted in multiple phases, with qualitative and quantitative methodologies. Content development used nominal group process, expert narratives, and literature review. After consensus was reached on content and scoring, item reduction began. Item retention and deletion decisions were based on a combination of practical considerations during field testing, expert and experienced clinician opinion of clinical relevance, and statistical information. Item retention or deletion decisions were based in part on the internal consistency analysis of the 19-item SBS based on 256 scores, which included participants who were apparently healthy and those with pathology. We applied a decision matrix to yield the final 11-item version. The 11-item SBS demonstrated good internal consistency (α = 0.762), intrarater rater reliability (ICC(3,1) = 0.96 to 0.99), and interrater reliability (ICC(2,1) = 0.87). On the basis of analysis of 156 scores, construct and concurrent validity were established. The SBS appears to meet the criteria required to make it a useful tool. The SBS can be used to assess sitting balance in frail individuals who are primarily nonambulatory who might exhibit floor effects for traditionally used balance measures. The SBS is the first attempt to assess lower balance abilities. Further assessment of the tool's validity with larger and varied samples is necessary.