The baby is due this summer, said publicist Evelyn Karamanos.
MacLachlan, 48, starred in "Twin Peaks," David Lynch's short-lived ode to bizarre doings in the Northwest. His TV credits also include HBO's "Sex and the City" and ABC's "Desperate Housewives."
Gruber is an executive producer on Bravo's "Project Runway."
The couple have been married for six years.
Karamanos made the announcement Friday.