Valle had represented District 2, which includes Hayward, Union City, Newark and parts of Fremont, since 2012. His cause of death was not immediately available.
Supervisor Nate Miley issued a statement about Valle's death, saying he "was an advocate for at-risk youth, job development programs, and was a steadfast champion for organized labor."
Oakland City Councilmember Kevin Jenkins also wrote about the supervisor's death, saying on social media that Valle was a "dedicated public servant. His passion for environmental and social justice was unparalleled, and his commitment to community will endure."
Valle is the second member of the five-seat Alameda County Board of Supervisors to die in office in recent years.
District 3 Supervisor Wilma Chan was hit and killed by the driver of a vehicle in the city of Alameda in November 2021. Following her death, the board appointed Chan's former chief of staff Dave Brown to replace her, and Lena Tam was elected to the seat in last November's election.