State-space models are used to examine long-term changes in the seasonal amplitude and phase of upper ocean temperatures from a set of time series representing the meridional and offshore extent of the California Current System (CCS). We use global one-degree summaries from the World Ocean Database at 11 locations and 10 standard depths in the upper 200 m for the period 1950--1993. The seasonality of upper ocean temperature in the CCS is highly nonstationary, with significant interannual to decadal changes in seasonal amplitude and phase apparent over the period of study. The 1950s and early 1990s were characterized by high seasonal variability in upper ocean temperatures, while the intervening years were characterized by a reduced seasonal cycle. Long-term changes in phase were also observed, with seasonal extrema occurring 1--2 months earlier in the year by the 1990s. The leading common seasonal components, dominated by the longer (yearly, 6-month, 4-month) periodicities, explain most of the seasonal fluctuations in the temperature series and partition the variance into well-defined dynamic regions. In particular, long-term changes in seasonality at 30--75 m in the major coastal upwelling centers (34¿N--38¿N) differed from that observed north of Cape Mendocino, within the Southern California Bight, and farther offshore. The observed spatial patterns suggest that the changes in temperature seasonality off central California reflect a significant low-frequency modulation of the intensity, timing, and duration of coastal upwelling in the CCS. This nonstationarity of the seasonal temperature cycle is superimposed on both a region-wide warming trend and spatially heterogeneous responses to low-frequency climate events, such as regime shifts and El Ni¿o events. Results from this study demonstrate that the character and potential biological impacts of climate variability can only be fully realized by considering a nonstationary, nondeterministic seasonal cycle.