Authorities said the fire began in a second-floor bedroom in the 2600 block of 55th Way at about 1:30 a.m. Everyone in the house was able to get out except the toddler.
Neighbor Patricia Hubbard said she heard the boy's family frantically screaming for help.
Charles Hakopian, 25, was one of the firefighters who ran inside the burning building to help search for the toddler, but he ran into some trouble.
Capt. Rich Brandt of the Long Beach Fire Department said the fire had been burning between the floors for awhile, and when Hakopian made entry, the floor gave way, and he fell into the hole.
Hakopian caught himself and was able to pull himself from the hole, and then continued to search for the boy, finding him in a closet in a back bedroom.
"When they brought the baby out, the baby was all like out," Hubbard said.
Paramedics got him breathing again on the way to Long Beach Memorial Medical Center. The boy is in stable but critical condition.
The firefighter had bumps and bruises, but he did not need to be hospitalized.
Brandt said Hakopian's quick thinking and sheer determination saved the toddler's life and is nothing short of heroic.
"Charles is just another example of our aggressive firefighters and what we do on a daily basis," he said. "We're glad that we can make a difference, and today, it paid off very well."
Officials believe the flames were sparked by an electrical shortage downstairs.