Thermal microclimate in intertidal tidepools is driven largely by abiotic conditions (i.e., sun exposure, tidal height, air temperature, water temperature), but algae substrate cover can substantially ameliorate these conditions. In this study, we measured the thermal inertia of all seaweeds found at ten tidepool locations across Southern California in June 2022-January 2023 (ranging across 35.2589°N, 120.8975°W to 32.7269°N, 117.2562°W). A representative sample of each seaweed (no senescing portions; stalk must be included) at each site was collected by hand and transported in a cooler with water sourced from the site to Chapman University in Orange, CA. Transport time did not exceed 5 hours. We also collected a few rocks and a sand sample as control measurements. Once in the lab, the seaweeds were photographed and their wet weight and volume displacement were measured. Seaweeds were placed under a heat lamp for 30 minutes and their surface temperature was measured using a thermocouple probe every 10 minutes starting at time=0. Mass was measured at each time point as well as a control for water loss. Seaweeds were dried for 48 hours at 60°C for a final dry weight. Measurements of mass were used to calculate thermal inertia, which is the change in temperature over a set period of time as a function of mass.