Judge Juan Carlos Peinado, who is leading the inquiry, maintained Sánchez's hearing for next Tuesday at 11:00 am (0900 GMT), according to a court filing seen by AFP.
He said he had summoned Sánchez as the spouse of Begoña Gómez, not as prime minister -- which would have allowed the Socialist premier to testify in writing as he had requested.
The judge is scheduled to question Sánchez at the premier's official residence.
But Sánchez can testify at a later date in writing about "relevant facts of which he has had knowledge by reason of his position", the judge added.
The only other time a sitting Spanish prime minister had to testify in a judicial case was in 2017 when Mariano Rajoy was summoned in a graft case that led to the conviction of several members of his conservative Popular Party (PP).
Gomez is being investigated for alleged influence peddling and corruption following a complaint filed by an anti-graft NGO with links to the far-right called "Manos Limpias" - Spanish for "Clean Hands".
Sánchez claims harassment
Sánchez has denied any wrongdoing by his wife, repeatedly dismissing the allegations as part of a smear campaign against his government.
Sánchez can appeal the judge's ruling that he must testify in person, or he can choose not to testify.
His wife invoked her right to remain silent under questioning by a judge earlier this month.
Gómez, has worked in fundraising for years, notably for foundations and NGOs. She is alleged to have used her husband's position as leverage within her professional circles, notably with businessman Juan Carlos Barrabés who was seeking public funding.
In his testimony, Barrabés - who teaches part of a master's course at Madrid's Complutense University that is run by Gómez - acknowledged meeting her five or six times at Moncloa, the premier's official residence.
Sánchez, in power since 2018, was also present at two of those meetings, he said.
Barrabés -- who got two letters of recommendation from Gómez before pitching for a public tender worth several million euros - said they only talked about matters of innovation, judicial sources said.
When the probe was opened in April, Sánchez shocked Spain by saying he was considering resigning over what he denounced as a campaign of political harassment by the right.
He took five days to reflect but ultimately decided to stay on.
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