Autonomous vehicles (AVs) are expected to have a great impact on mobility by decreasing commute time and vehicle fuel consumption and increasing safety significantly. However, there are still issues that can jeopardize their wide impact and their acceptance by the public. One of the main limitations is motion sickness (MS). Hence, the last year's research is focusing on improving motion comfort within AVs. On one hand, users are expected to perceive AVs driving style as more aggressive, as it might result in excessive head and body motion. Therefore, speed reduction should be considered as a countermeasure of MS mitigation. On the other hand, the excessive reduction of speed can have a negative impact on traffic. At the same time, the user's dissatisfaction, i.e., acceptance and subjective comfort, will increase due to a longer journey time. Therefore, additional approaches to mitigating MS should be considered without affecting journey time, such as vehicle and seat suspension designs. In this direction, this article investigates a novel active seat suspension (ActiveK) that operates according to K-seat. The K-seat is a novel passive isolator with negative stiffness (NS) elements proven to enhance comfort, but has difficulties in design, which can be overcome with ActiveK. The ActiveK-seat is benchmarked against a passive seat model, a semi-active model with a continuously controllable electromagnetic damper (EMD), and a simple active model. The seat models are not only compared with regard to comfort but also for their ability to mitigate MS while the vehicle is driving on a real road path with a Class C road roughness. The results are very promising and show up to a 70% and 25% decrease in metrics for discomfort and MS incidence, respectively, compared to the rest of the seat model.