Judge Peter Smith ruled that Janan Harb was entitled to more than £15 million ($23 million/21 million euros), plus the deeds to two plush London flats, thought to be worth £5 million each.
He accepted that the absolute ruler's son Prince Abdul Aziz bin Fahd had struck such an agreement with Harb, honouring the king's promise that she would be looked after for the rest of her life.
Unless the prince decides to challenge the ruling in the higher Court of Appeal, he will have to pay up within 28 days.
Born to a Christian Palestinian family, Harb, 68, is now a British national.
She says she was secretly married to Fahd in 1968 when she was 19 and he was a prince and the interior minister.
Harb claimed Fahd, who became king in 1982 and died in 2005, had promised to provide for her financially for the rest of her life.
She claimed Abdul Aziz told her in 2003 that he was prepared to honour his father's promise, and had offered to give her £12 million plus the deeds to the two flats by the River Thames.
She took legal action after claiming she had received neither the money nor the properties.
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"This has been 12 years of misery for me. I am very happy with British justice," Harb said after the ruling.
- 'Discreet ceremony' -
During a seven-day hearing in July, Harb said Fahd's family was against their relationship due to her Christian upbringing.
"Fahd was concerned about how this would be viewed by the Saudi public, many of whom follow a strict interpretation of Islam that preaches deep enmity and hostility to all other religions," she said.
"It was for this reason that in March 1968 we underwent a discreet ceremony of marriage."
Harb said she converted to Islam shortly before the wedding.
She said she fell pregnant three times in the following three years but had abortions "because of his concern over the outcry" he might have faced.
She said she was banished in 1970. She had two further marriages, which both ended in divorce.
Little is known about king Fahd's private life, but he was known to have had multiple wives.