One of South Korea's most powerful white collar criminals, Lee was twice imprisoned for bribing a former president.
South Korea's government justified the move, saying the de-facto leader of the country's biggest company was needed back at the helm to spearhead economic recovery post-pandemic.
This marks another swing in a struggle over how the country is run that has raged since mass protests took over Seoul six years ago and ousted a president from office.
Lee's crimes were directly tied up in the corruption scandal that led to the imprisonment of former president Park Geun-Hye, in office from 2013-2017.
The "Crown Prince of Samsung" - as he was dubbed by protesters - paid $8 million (£6.6m) in bribes to President Park and her associate to secure support for a merger opposed by shareholders that would shore up his control of his family's empire.
When it was revealed, millions of South Koreans turned out at candlelit protests every weekend in the 2016/2017 winter, demanding an end to Park's government and the stitch-up between politics and business.