HONG KONG—A pair of billionaire brothers who run one of the world’s biggest property companies met different fates in the city’s highest-profile corruption trial in decades, with one walking free and the other potentially facing years in prison.

Hong Kong’s former No. 2 official was also convicted in the case, which drew attention to the tight relationship between the government and property moguls in the city, which has the world’s highest housing prices.

Thomas Kwok, the co-chairman of Sun Hung Kai Properties Ltd., was found guilty of conspiracy and was immediately taken into custody after the Friday verdict. Younger brother Raymond Kwok walked free into the overcast late afternoon and said he was very sad that his brother was convicted. He said he would continue to support his brother. “I’m very grateful for the support of my wife and children,” Raymond Kwok said.

The city’s former No. 2 official, Rafael Hui, 66 years old, was convicted of five charges, many of them tied to the payments that prosecutors said were designed to make him “favorably disposed” to the Kwoks’ interests.

Mr. Hui is the most senior official to have been found guilty of misconduct since Britain handed Hong Kong over to China in 1997, while Thomas Kwok is a scion of one of the city’s leading tycoon families, responsible for large swaths of the city’s glittering skyline, including its three tallest buildings.