The court affirmed a lower court's 2012 ruling that Sondhi Limthongkul violated Securities and Exchange Commission regulations in 1996-97 by presenting fraudulent financial data to obtain a large bank loan for one of his businesses.
He was also accused of not informing the Stock Exchange of Thailand and the board and shareholders of the company guaranteeing the loan, which fell into default.
Two of his associates in his Manager Group who were involved also had their 20-year sentences affirmed today.
Thaksin was then ousted in a military coup, but his supporters and opponents continue to contend for power.
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Sondhi was one of a generation of Thai business entrepreneurs who built their empires during the boom years of the late 1980s and early 1990s, taking advantage of a loose regulatory climate.
He expanded a mostly print-based media empire focusing on business coverage and diversified into other fields, including satellite communications and tourism.
Sondhi, whose holdings were overextended and highly leveraged, was struck hard by Asia's 1997 financial crisis, which devastated even established Thai banks and businesses.
Thaksin, who had already entered politics by that time, retained most of his fortune and used it to build his own political party, which won the 2001 general election and put him into the prime minister's seat.
Friendly relations between the two men soured as Thaksin failed to carry out appointments and actions that Sondhi had sought, and by late 2005, Sondhi was leading small rallies attacking Thaksin.
It was often called the Yellow Shirts because it adopted the royal color.
Army and palace disenchantment with Thaksin took advantage of the pressure to stage a coup in September 2006.
Thaksin's political allies regained power in a 2007 election, and the People's Alliance for Democracy became more militant in 2008, taking over the prime minister's office for three months and Bangkok's two airports for about two weeks.