The far-right National Front party also gained its first two Senate seats in the balloting.
The Senate press office said a final count will await tallies from all of France's overseas holdings not likely until tomorrow, but it was already clear that the conservatives were back in control of the 348-seat chamber.
A conservative win was expected, Senators are chosen by nearly 88,000 electors nationwide, including local officials who took office in a conservative victory in municipal elections six months ago.
The showing confirms pockets of strong popularity for the National Front and a political turnaround for France's recently embattled mainstream right, which was riddled with internal squabbling months ago and is now riding high with the return of former President Nicolas Sarkozy to the politic scene.
The right has benefited from widespread frustration with Hollande's persistent, but failed, efforts to revive a moribund economy and bring down entrenched double-digit unemployment rates.
Half of the Senate is elected every three years. Senators hold six-year terms. Under the French constitution, the chamber's president selected by its majority is first in line to become France's president if the head of state dies or resigns. A battle for that post is expected this week.
