Most of the annual precipitation over California occurs in a short window between November and March and is highly dependent on the frequency of winter storms. Thus, the amount of precipitation can vary strongly from year to year, as demonstrated in the most recent decade (2012-2022), when year-to-year swings between high and low winter precipitation extremes resulted in drought, wildfires, and floods throughout California that caused billions of dollars in damage. We show that instrumental precipitation and streamflow data (1940-2019) across the state show an increasing trend in variability starting in the 1950s that is primarily caused by intensified wet extremes. Moreover, we examine this trend in the context of the past 600 years based on 20 tree-ring based hydroclimate reconstructions throughout the state. We show that there is a synchronous rise in precipitation and streamflow variability in the 20th century over California that has reached unprecedented levels over the past 600 years, and that there is another, comparable period in the 16th century. The major difference between these two periods is that the current increase in variability is driven by wetter extremes, while the 16th century trend was driven by drier extremes. Our results are consistent with climate model simulations that suggest an increasingly volatile future for California’s hydroclimate, especially as wet extremes intensify further. Thus, California’s average climatic conditions will become less important to the state’s overall hydroclimate, as most past and future change has and will be driven by changes in the nature of extreme events.