The boy's mother, 26, was pulled alive from the hole by her traffic police officer husband, reports say.
But he was unable to rescue his son, who was thrown from his pushchair and fell into the sewer below.
"The woman managed to grab on to the edge of the collapsed pavement, which saved her life," a Bryansk official was quoted as saying.
Her husband had apparently been on duty in the area on Sunday afternoon and rushed to the scene, climbing down into the hole with the aid of a rope to pull his wife to safety.
Named by Russian media as Tatyana Didenko, she was taken to hospital in shock.
Although remnants of the one-and-a-half-year-old boy's clothing were found by search teams, it was not until late on Monday afternoon that his body was discovered.
The toddler, named by reports as Kirill, was found 6km (3.5mi) from the spot where he disappeared, according to police.
The cause of the pavement collapse is unclear but investigators are looking at problems with the construction of a local sewage plant.
Bryansk regional governor Nikolai Denin told Russia 24 TV that recent mild weather could have caused a pipe to rupture, leading to sand below the pavement being washed away.
Investigators have opened a criminal manslaughter case.
Mr Denin said a large proportion of the city's infrastructure was not in top condition: "We have to start rebuilding things because otherwise this could happen again.