Fatality at such a young age “is a very rare occurrence,” said government spokesman Dr Emmanuel Andre, adding that her death “shook us”.
The girl had had a fever for three days before her death, and tested positive for COVID-19, said another spokesman, Steven Van Gucht.
No other details were given about her case, including whether she had any other underlying health problems.
It was the first death of a child in the coronavirus crisis in Belgium, which has now recorded a total 705 deaths from the disease it causes, according to the latest official toll.
Last week, France reported the death of a 16-year-old girl from coronavirus in the greater Paris region.
Although serious COVID-19 infections are uncommon among the young, some exceptional cases have been taken to hospital intensive-care wards, as US health authorities have pointed out.
Belgium’s toll on Tuesday represented a jump of nearly 200 fatalities from that given the previous day, which stood at 513.
It comprised 98 deaths recorded in the preceding 24-hour period, plus another 94 deaths over previous days but which had not been counted in the national tally, Andre said.
The small EU country, with a population of 11.4 million, now has 12,775 cases of persons tested positive for COVID-19, of whom 4,920 have been hospitalised, including 1,021 in intensive care.
Hospitals in Brussels and in two provinces, in the north and the west, are now confronted with “a more complicated situation” as beds
fill up, Andre said.