Labour appointment puts French women ministers in majority
El Khomri, previously a junior minister with responsibility for urban affairs, is seen as close to Valls and President Francois Hollande.
She replaces Francois Rebsamen, who has resigned to return to his previous role as mayor of Dijon.
Rebsamen kept a low profile after his appointment in the reshuffle that made Valls prime minister in 2014, but Hollande has pledged to reduce joblessness, stuck stubbornly above 10 percent, or stand aside as the Socialist party candidate for re-election in 2017.
El Khomri brings the number of women among Valls' 16-strong team of senior ministers to nine.
(Reporting by Andrew Callus; Editing by James Regan)