Urban Heat Island (UHI) is a worldwide threat affecting building energy demand, public health, and energy security. Green wall deployment can simultaneously positively impact UHI and building energy demand depending on climate zones. According to the different climate zones worldwide, the present systematic literature review (SLR) investigates the direct effects of green wall installation on building energy use and UHI. 1325 articles were screened, and 51, corresponding to 647 case studies, were selected after removing those with methodological or statistical heterogeneity. The effects of green wall deployment have been explored according to cooling and heating season, weather conditions, daytime, nighttime, green wall typology, green wall orientation, and application scale. The performed analyses show that green walls: (1) can reduce heating and cooling building energy demand up to 16.5% and ∼51%, respectively, and mitigate UHI up to ∼5 °C in all the investigated climate zones; (2) can decrease to the greatest extent building energy needs when applied in low-density urban contexts where they can be installed on the entire building. Besides, when applied to a single façade, South orientation should be preferred in most climate zones to maximize building energy saving; (3) have the best UHI mitigating potential—up to 8 °C—in highly urbanized areas featured with narrow streets surrounded by high-rising buildings. Altogether, green walls are a fit-all solution to reduce building energy demand and mitigate UHI, providing healthier living conditions. However, further research is necessary to include quantifiable and unquantifiable effects omitted in the current study. •Green walls have been investigated in different climate zones.•The effect of green wall deployment on building energy use and UHI was explored.•Green wall deployment decreases building energy needs for space heating and cooling.•Green wall installation may reduce air temperature by up to 8 °C in street canyons.•Large-scale installation of green walls may fully mitigate UHI.