Indoor environmental quality is an important determinant of comfort, health, and productivity in office buildings, particularly in tropical climates where high temperature, humidity, and inadequate ventilation are common. This study evaluates thermal comfort and ventilation in selected administrative offices within Wuye Ultra-Modern Market, Abuja, Nigeria. A field-based, multi-method case study approach was adopted using indicative environmental measurements, tissue deflection airflow testing, building observation, sun-path assessment, window-to-wall ratio analysis, and occupant questionnaires. Five offices — Administrative, Accounts, ICT, Facility, and Managing Director's — were studied. Average indoor temperatures ranged from 25.9°C to 29.3°C and relative humidity from 78.0% to 85.3%, both exceeding recommended comfort limits. Window-to-wall ratios ranged from 10.5% to 16.7%. Tissue testing confirmed cross-ventilation in the ICT, Accounts, and Facility offices through west-facing openings, while the Administrative and MD offices showed negligible airflow due to exclusively lobby-facing windows. Questionnaire results showed 46.1% of occupants felt warm, and all respondents identified the afternoon as the hottest period. The study concludes that thermal discomfort is driven by high humidity, afternoon solar heat gain, top-floor exposure, internal-facing windows, and absent cross-ventilation, and recommends reconfigured openings, external shading, roof insulation, and coordinated mixed-mode ventilation.