The case caused a scandal as it involved several prominent men in the city state.
Juerg Buergin, 41, was found guilty by a district court of two charges of having sex with a 17-year-old call girl and will be sentenced on May 8.
Judge Eddy Tham said Buergin had failed to disprove the charges that sexual intercourse between him and the girl took place on two occasions in 2010 and 2011.
Buergin, a married former executive with banking giant UBS, had said at the start of his trial in March that he was tricked into thinking the girl was older than 17.
Prostitution is legal in Singapore but it is against the law to pay for the sexual services of girls under 18, in a bid to protect the young from exploitation.
The case caused an uproar last year after Buergin and 50 other men were accused of paying for sex with the girl.
One of them was socialite and environmental activist Howard Shaw, 41, a grandson of Asian movie mogul Runme Shaw.
Buergin, who arrived in court Tuesday wearing a dark business suit, was calm and showed no visible reaction after the verdict was read.
State prosecutors asked for a sentence of "not less than six months" for each of the two charges.
Court documents show that Buergin booked the prostitute, who went by the name "Chantelle" on an escort agency's website, on two occasions in September 2010 and January 2011.
He met her at luxury hotels for paid sex.
Buergin made the bookings under a false name "Robert James" but the mobile phone number he used was traced to UBS, which had assigned the number to him, according to the documents.