We are in the era of Industry 4.0 with the world going towards everything, everywhere, and anytime with “things” being enablers of technology. Our world is becoming smarter with everything (mobile phones, cars, TVs) connected to each, having unique addresses and communication mechanisms. It is foreseen that by 2025 every little thing, like pen, paper, food packages, etc., would be operated over the internet by internet of things (IoT) towards a smarter world. However, in order to enable the smart world to be sustainable, IoT should be embarking into energy efficient (green) paradigms. Since IoT is also a key enabler for smart cities, a green-aware design of smart cities could see a potential to create a green IoT ecosystem for smart cities. This ecosystem would comprise the green IoT lifecycle, the six pillars of smart cities, and focusing on the green infrastructure, green applications, and green energy. This conceptual study would motivate researchers embarking into smart city projects and keeping green concepts intact in their design and implementations.