Firefighter Vince Cruz was posthumously promoted to the rank of fire lieutenant by the department following his death.
"I worked with him all my career," Acting Fire Chief Joey San Nicolas said in a statement. "He was a dedicated first responder. Just like the rest of the men and women here, nothing was going to stop him from helping people in need, even if it meant major sacrifices.
"Vince made the ultimate sacrifice. He gave his life so that he could help others. We're just in shock right now. We're really going to miss him."
Acting Governor Ray Tenorio declared the state of mourning and honored Cruz for his dedication.
"He literally spent his life helping people, up until the day he died," Acting Governor Tenorio said. "This is a very difficult time for (his family) and the fire department. We ask the community for their prayers and support for Vince's family, friends and fellow firefighters."
Funeral arrangements are pending.
Guam is an organized, unincorporated territory of the United States located in the western Pacific Ocean.