This article proposes a new performance index called Transit Service Indicator (TSI), which could be used as a comprehensive measure for quantifying the quality of service of a transit system. TSI integrates multiple performance measures (e.g., service frequency, hours of service, route coverage, and travel time components) within a systematic framework. It takes into account spatial and temporal variations in travel demand, recognizing that quality of service is a result of interaction between supply and demand. A case study is conducted to examine the sensitivity of the proposed TSI to various system design and condition variables and parameters.