This document presents a novel adaptive myoelectric decoding algorithm to improve long-term accuracy of prosthetic limb control. The algorithm relies on unsupervised updates to the training set to adapt to both slow and fast changes in myoelectric signals over time. An able-bodied user performed eight wrist movements over 4.5 hours while EMG data was collected. The proposed algorithm maintained decoding accuracy with a decay rate of 0.2 per hour, compared to 3.3 per hour for a non-adaptive classifier, demonstrating its ability to adapt to changes in signals and improve reliability of myoelectric prostheses.