The iPhone 6S and iPhone 6S Plus (stylized and marketed as iPhone 6s and iPhone 6s Plus) are smartphones that were designed, developed, and marketed by Apple Inc. It is the ninth generation of the iPhone. They were announced on September 9, 2015, at the Bill Graham Civic Auditorium in San Francisco by Apple CEO Tim Cook, with pre-orders beginning September 12 and official release on September 25, 2015. The iPhone 6S and 6S Plus were succeeded by the iPhone 7 and iPhone 7 Plus on September 7, 2016 and were discontinued with the announcement of the iPhone XS, iPhone XS Max, and iPhone XR on September 12, 2018.

The iPhone 6S has a similar design to the but includes updated hardware, including a strengthened chassis and upgraded , a 12-megapixel rear camera that can record up to video at 30fps, improved support, and "Hey Siri" capabilities without needing to be plugged in. The iPhone 6S also introduces a new hardware feature known as , which enables pressure-sensitive touch inputs.

The iPhone 6S had a positive reception. While performance and camera quality were praised by most reviewers, the addition of 3D Touch was liked by one critic for the potential of entirely new interface interactions, but disliked by another critic for not providing users with an expected intuitive response before actually using the feature. The battery life was criticized, and one reviewer asserted that the phone's camera was not significantly better than the rest of the industry. The iPhone 6S set a new first-weekend sales record, selling 13 million models, up from 10 million for the iPhone 6 in the previous year. However, Apple saw its first-ever quarterly year-over-year decline in iPhone sales in the months after the launch, credited to a saturated smartphone market in Apple's biggest countries and a lack of iPhone purchases in developing countries.