Few studies have documented chiton growth rates, and lifespans for most chiton species remain unknown. This study presents the first size-at-age and longevity estimates for Cryptochiton stelleri and Katharina tunicata, two of the largest known species of chitons. Age estimates were based on growth ring counts obtained by cross-sectioning and polishing valves. This process alone showed clear growth rings in shell plates of K. tunicata. Valves of C. stelleri did not show conspicuous growth rings in cross section, so the acetate peel technique was used to highlight growth bands. Linear growth in terms of body length and shell width approached an asymptote in both species and was described with the von Bertalanffy growth function. Both C. stelleri and K. tunicata displayed indeterminate growth with regard to tissue weight and shell weight, continuing to maintain a steady growth rate throughout their lifespans. The oldest K. tunicata specimen was estimated to be 17 years old. The oldest C. stelleri was estimated to be 40 years old, making it the longest-lived chiton species known.