The woman, surnamed Yang, extorted 300 million won (US$200,000) from Son last year by sending him a foetal ultrasound image and threatening to go public.
She reportedly splurged the cash on luxury and designer goods, before her and an accomplice surnamed Yong tried to extort a further 70 million from the Los Angeles FC star.
Son contacted the police, leading to their subsequent arrest and indictment.
On Monday the Seoul Central District Court gave a four-year term to Yang, who is in her 20s, for blackmail, local TV said.
Co-conspirator Yong — with whom she was reportedly in a romantic relationship — was given a two-year term for attempted blackmail, Yonhap news agency said.
The two have been in custody since May.
Judge Im Jeong-bin said on Monday that Yang had taken “extreme measures” in her bid to tarnish Son’s reputation.
“It was not limited to simple threats or demands for money — she went further by taking action, such as informing the media and advertising agencies, exploiting Son’s status as a public figure,” Im said, according to Yonhap.
Son “suffered considerable psychological distress as a result of the case becoming public”, he added.
The South Korean football captain appeared at a closed trial session and gave testimony as a witness last month, local media reported.
It remains unclear whether Yang — who local media have speculated terminated the alleged pregnancy — was actually carrying the star footballer’s child.
“After learning about her pregnancy, Yang did not verify whose child it was,” Judge Im said.
“Although Yang said she believed the fetus was Son’s, her testimony was inconsistent and therefore difficult to accept,” he added.
The 33-year-old former Tottenham captain Son was in October named the second-highest paid footballer in Major League Soccer.
Son arrived in California in August after the most expensive transfer in MLS history, estimated at $26 million.
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