The figures translate to 57 deaths per every 1,000 live births, down from 88 deaths in 1990.
The U.N. Children's Fund and the U.N. World Health Organization, which released the figures, said the rate decline means about 12,000 fewer children die each day than did two decades ago.
The reports cited greater access to healthcare, particularly in remote areas, broader immunization coverage and higher-quality care in many countries among factors for the improvement. The agencies said child mortality rates are dropping in every region of the world, including areas with the highest number of under-5 fatalities. Success stories include Niger, Malawi, Liberia, Timor-Leste, Sierra Leone, Nepal and Bangladesh.
Experts, however, noted current mortality rates -- which come to about 21,000 deaths every day -- are still too high. They said these children die of preventable causes such as neonatal sepsis, pneumonia, and diarrhea.
Half of all deaths last year of children under 5 occurred in just five countries -- India, Nigeria, the Democratic Republic of the Congo, Pakistan and China.
"Focusing greater investment on the most disadvantaged communities will help us save more children's lives, more quickly and more cost effectively," he said.