Marine heatwave (MHW) events are emerging as dominant and disruptive extreme disturbance events on the Pacific coast. These events are expected to have significant impacts on shellfish species and aquaculture operations. In general, thermal stress induced by MHW events has been documented to have impacts on the productivity and biodiversity of coastal marine ecosystems in California. Yet, there is limited understanding on how thermal stress will affect economically important shellfish species, especially at vulnerable early life stages. This study assessed the effect of high temperature stress that resembled recent local MHW conditions on the larval stages of the Kellet's whelk, Kelletia kelletii, an emerging seafood species from California temperate reefs. Adult whelks were collected at temperate reef sites in the Santa Barbara Channel, and larvae were sourced from egg capsules laid by adult whelks in the laboratory. Thermotolerance trials were determined for two stages of K. kelletii larvae: encapsulated veligers and veligers that had emerged from the capsules as hatchlings. During the trials, larvae were scored for two metrics: (1) mortality and (2) developmental abnormality. After exposure of larvae to a range of temperatures (15°C–37°C) in acute thermotolerance trials (1 h), the mortality of veligers and hatchlings occurred at similar temperatures. In contrast, temperatures that induced developmental abnormalities for both encapsulated veligers and hatchlings were lower than temperatures that caused mortality. And further, temperatures that induced abnormal development were different for the two stages. These results provide some of the first insights on effects of environmentally relevant MHW temperatures on larval forms of the Kellet's whelk, and suggest the potential decline of populations in situ in response to continued stress from local MHW events.