RN presidential candidate Marine Le Pen hailed a "historic victory" after the results were announced.
Local elected officials vote for new senators for half the upper chamber every three years, each to serve a six-year term.
The far-right party entered the Senate for the first time with two seats in 2014, when it was known as the National Front, and won three seats in 2023.
Le Pen is eyeing her best chance yet at power in elections next year when centrist President Emmanuel Macron steps down. "This is a historic victory, and a major milestone with the formation for the first time of a group in the Senate," she said on X.
Nine RN senators were elected on Sunday, on top of the three who won seats in 2023.
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A further two were elected from RN ally Eric Ciotti's Union of the Right for the Republic (UDR) party.
RN president Jordan Bardella said the party's senators would form a group with UDR allies.
Ciotti quit the right-wing Republicans (LR) party in 2024 after calling for an alliance with the far right.
The upper chamber's LR president of more than a decade, 77-year-old Gerard Larcher, is expected to be re-elected next week.
Le Pen's anti-immigration party emerged as the single largest party in the lower-house National Assembly after Macron called snap polls in 2024, though it fell well short of a majority.
(With newswires)
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