Non-enclosed atriums, which give unobstructed access to the surrounding area, are gradually becoming common in modern shopping malls. The environmental problems they cause (e.g., vertical thermal stratification and overheating) are thus affecting adjacent occupied zones more often. This study focuses on the thermal environment of occupied zones adjacent to non-enclosed atriums and investigates influencing factors. Field tests of heat source intensity, air infiltration rate, and indoor temperature distribution were conducted in a typical shopping mall with multiple non-enclosed atriums. The vertical temperature gradient index (k) was adopted to illustrate temperature distribution among different layers where the atrium gives access straight through, which varies considerably in the presence of various heat sources. According to test results, both solar radiation transmitted and heat conducted through the glass ceiling of such an atrium contribute to the thermal stratification of indoor spaces, with the temperature gradient being 50% higher than in spaces without glass ceilings. As has been confirmed, air infiltration continuously introduces heat into the indoor environment. During summer, when air-conditioning operates, outdoor air infiltration occurs at rooftop entrances (approximately 3.0 m² opening area), with an average flow rate of over 20,000 m³/h, accompanied by heat of 94–213 W/m². The thermal stratification caused by non-enclosed atriums has a corresponding impact on occupants’ thermal sensation in the adjacent zones, as qualitatively verified by a subjective survey. Given this significant influence of non-enclosed atriums on the surrounding thermal environment, the study suggests weakening heat intensity through atrium form, glass ceiling, and air infiltration.