The 35-year-old confirmed her decision in a statement posted to her Twitter feed and said that, though it has been "a great honour to compete for Great Britain", the time had come to hang up her spikes.
Onuora, who was born in Liverpool and is the sister of former footballer Iffy Onuora, won 4x400m bronze at the Rio Olympics in 2016 having won the same medal at the World Championships a year earlier.
She competed for the first time internationally in 2006, reaching the semi-finals of the 100m and winning the 4x100m relay, before going on to win medals at the Manchester Commonwealth Games in 2009 and European Athletics Championships.
Her build-up to the 2016 Olympics was severely interrupted when she contracted Malaria during a trip to visit family in Nigeria, temporarily leaving her unable to walk, yet she recovered to win bronze with Team GB in Rio.
"Competing as an elite athlete has allowed me to experience many incredible things I never thought I would imagine," she said in her statement.
"Amongst the many other medals I won, being one of only a handful of British athletes to complete 'the full set' - becoming a European, Commonwealth, World and Olympic medallist - makes me immensely proud.
"I'm forever grateful for all the people who allowed me to fulfil a big dream that a young black girl from Liverpool was able to achieve.
"Every setback and low I experienced was worth it to get to those special moments that will always hold dear to me."